Last minute trip to Washington D.C.

Check out the Basilica of the National Shrine of the Immaculate Conception (Brookland Metro Station) - it really is an amazing place architecturally and otherwise (even for non-Catholics). It's right by Catholic University and the John Paul II Center, which is also very interesting (as is the Franciscan Monastery, also nearby). There are tours of all three places, or you can view them on your own.
https://www.nationalshrine.org/visit/
https://www.jp2shrine.org/en/index.html
Just check the event calendars ahead of time to make sure there are no graduation or other ceremonies going on (if you're visiting in May).

If you have a car with you or are flying into IAD (Dulles), then see the Udvar Hazy Air and Space Museum almost next door. To me, it's more interesting than the one on the Mall.
https://airandspace.si.edu/udvar-hazy-center

There are also non-Smithsonian Museums that are a bit off-the-beaten-path like the Phillips Collection and Dumbarton Oaks and Gardens. Both are excellent.

I lived in DC for many years and my #1 recommendation I give people is to take the Dupont Circle and Embassy Row tour offered by this company: https://www.dcdesigntours.com/. I’ve done it twice with visitors and had fantastic experiences. Gives a great taste for a less dry side of DC history and a view into a beautiful neighbourhood to boot!

The Newseum is my favourite museum, and I think the Library of Congress has the most gorgeous interior. It can be fun to rent pedal boats on the Tidal Basin or paddle boards on the Potomac. Union Market or Eastern Market for food and local vibes.

As a native, I highly recommend family-friendly, bustling (with pedestrian traffic), walkable and lively Old Town Alexandria. Always changing, never dull. It's on the Potomac just a few Metro/subway stops (King St. Station) south of DC. From the station to the Potomac at the end of King St. is about a mile. Tons of eateries, pubs, ice cream & coffee shops, bakeries, pubs, small, locally-owned retail, abounding with 18th century history, Irish and Scottish influences, a couple of cobblestone streets, etc.

The Torpedo Factory on Union St. at the river is a former - well, torpedo factory - turned into a multitude of arts and crafts workshops. A real torpedo greets you at the door. The city also runs a small archaeological museum there. From the dock behind it you can take a ferry to Maryland to see the "Awakening", an unusual in-ground-above-ground sculpture that's graced the area for decades.

Carlyle House, the Apothecary (18th century pharmacy), Gadsby's Tavern (era-themed lunches and dinners served on pewter plates, outdoor seating and the occasional strolling troubadour - reservations a must). In the day, it was an inn frequented by many of the movers and shakers of the time. The visitor's center at the corner of King and Fairfax Sts. has all the details on what to see and do in Old Town.

If you're outdoors-oriented, then rent a bike and ride the paved Mt. Vernon trail (18 miles from Roosevelt Island across from Rosslyn & Georgetown to George & Martha's place. It goes right through Old Town, along Union St.). The plantation is nine miles south of Old Town.

Cycling or walking on the C&O Canal Towpath, starting in Georgetown, is very pleasant. I've been away a few years, but the mule-driven barge once used to haul goods may by now have been restored and made water-worthy for short trips on the canal. It and the park (C&O Canal National Park, that is) are run by the National Park Service. You can also rent a canoe from a concessionaire on the towpath.

Space Window in the National Cathedral, Spy Museum, Vietnam Memorial, Riggs Library,
Architecture Museum a.k.a National Bldg Museum,
and Union Market.

One thing I did that was way cooler than I expected it to be was to get a Library of Congress library card. Basically you go to the ugly administrative building just around the corner from the LOC and get your picture taken and get a card. It only took about 15 minutes. With the card, this allows you to actually go in and use the LOC's stacks, not just stare at them from the plate glass window high above. It's a really pretty building architecturally. Bonus, the cafeteria at the top of the LOC admin building was affordable and tasty (not much else to eat around that area).

Also find out when the Supreme Court has the guided talks of the courtroom. We stumbled upon one just as it was happening, and they let you go in and actually sit in the audience chairs, and they point out where the justices sit and talk about the process. Really neat (and free). It's just down the street from the LOC.

Also going out to Mt. Vernon is well worth it.

Go to Arlington National Ceremony and witness the hourly changing of the guard at the Tomb of the Unknowns. I've been to DC 10 times or so, and always go to see that.

Great suggestions everyone! Keep’ em coming!

My 16 year old and I spent 3.5 days in DC over spring break. 2 things that we really enjoyed were 1) a monuments by night tour by Urban Adventures and 2)the American Indian museum. The tour we had planned-it was like a giant golf cart, so we could get up close and see everything in its nighttime glory without the huge bus crowds. Our guide/driver was named Monty and he's a DC native who knew alot of info and presented it well. The museum was an unplanned stop (I had heard they had good food and it was near lunchtime) and we spent about 2 hours. Very interesting, and the buffalo chili flatbread taco was yummy! We used a Streetwise Map from Amazon all the time-I have gotten them for Paris and Munich trips. Cheap, easily foldable, and laminated, they include a metro map as well as a street map with index for the major sites, hotels, etc. Plus, the DC Circulator bus is now free and routes go to many areas (like Georgetown) that aren't covered by the metro. Have fun!

You and your daughter might enjoy visiting the Belmont-Paul Women's Equality Museum. Out of the way and open when other spots aren't, it's pretty interesting. It's a quick visit you can make while waiting for the biggies to open. You can see the desk where Alice Paul wrote the Equal Rights Amendment. https://www.nps.gov/bepa/index.htm

To get to the Mall area from Dulles, you can catch the Silver Line Express ($5.00 cash when I rode it a couple years ago) then connect to the Metro Silver Line for the 30ish minute ride to the Mall. https://freetoursbyfoot.com/transportation-washington-d-c-dulles-airport/#rail

Have fun and try a meal at Hamilton or Old Ebbits.

Visit Mt Vernon. Absolutely fascinating.

In conjunction with a visit to Alexandria, VA, would be the boat trip up to Mt. Vernon, with a tour of that complex. That would be a very pleasant day (in good weather), and get you away from the urban, fast-paced and very touristy DC.
In DC, another recommendation for the Museum of the American Indian. Many interesting exhibits, often screening of videos, great gift/craft/book shop, and a wonderful food court. There are several sections with typical foods of different Native American regions: Plains Indians, Pacific Northwest Indians, Latin American Indians, etc. Each of you can pick and choose the dishes you like from the different stations — soups, salads, mains courses, desserts, etc. Plentiful, quite beautiful and tasty choices.
The Newseum is very interesting, but being private, admission ends up being quite expensive for a family, unlike the many Smithsonian and other public museums, which feature free admission. When I was there a few years ago, one family of 5 just walked away from the ticket counter after learning what their cost of admission would be.

To get to the Mall from Dulles via public transportation is relatively simple. Catch the Silver Line Express bus ($5 cash) to Wiehle-Reston Metro station. It's the only stop. Catch the Metro Silver line into the city, many stops. Check with your hotel to see what the nearest metro stop is. The mall is a deceptively large expanse and you don't want to get off at the wrong end of the Mall. As an alternative you can catch the Metro Bus 5A to either Rossyln, VA or L'Enfant plaza, both Metro stops. The same Metro card that you use on the underground metro works on Metro Bus. It won't work for the Silver Line Express.

The Korean War memorial is my favorite, especially early in the morning when there is still a little mist on the ground. The Martin Luther King Memorial is probably new since your last visit. I haven't been to the African American Museum, but I've heard it's amazing and well worth the time. See if you can get tickets on-line before you go. If not, get in line early. They release same-day tickets on a first come first served basis.

The Wharf is the newest "happening" place. You can get there from the Waterfront Metro stop (green line) or L'Enfant plaza (Green, Blue, Yellow, Orange or Silver lines). From there you can walk or take a free shuttle. Lots of restaurants, a couple of fire pits going at night and giant swings. There is also the Anthem, a performance venue at the Wharf.

The Kennedy Center does free performances on the Millennium Stage every night at 6. Check their website for the schedule. It's someone different every night. Prepare to get there early for popular acts. When it's full, it's full.

If you like modern (20th century and later) the Phillips Collection near Dupont Circle is good. There is also the National Museum of Women in the Arts. If you are a member of a museum at home, see if they are a member of a group like ROAM that allow reciprocity for entry fees.

There is a new stadium where the DC soccer team, DC United plays. I haven't been yet, but it's on my list. And the Nationals play near by. You haven't lived until you've seen the Racing Presidents live. Poor Teddy!

Re: transportation. I would take the bus or a taxi to the nearest metro as suggested. Twice I took a taxi all the way in and it took hours (and $$) because of the traffic. The metro is great.

The Newseum is one of my favorites; I think whomever curated their exhibits did a great job, but check to see if it is still open. Their location has been sold and the last I heard they hadn’t found a new home.

I also enjoy the botanical garden if you are in that area and some of the best food can be found in the cafeteria at the Indian Museum.

"Near the Mall" covers quite a large area. Give us the name and address of your hotel and we can proviide more accurate transportation info.

The International Spy Museum (which I've never been to) is relocating to L'Enfant Plaza. It's supposed to open there on May 12.

Many of my favorites have already been covered, so I'll just put in second (or third) plugs for Mt. Vernon, the Udvar-Hazy center, the Newseum, the Spy Museum, the Phillips Collection, and the Vietnam Memorial (you can't understand this one by seeing it from afar; you have to walk through it, to understand why its designer called it "a rift in the earth").

As for the Smithsonian, each one is huge, so be selective. They are free to enter, but allow time for the security lines. I like the American Arts and the Portrait Gallery (same building), as it's not on the Mall and so can be less crowded.

I never used to like Georgetown, because all I saw were the commercial strips on M Street and Wisconsin Avenue. That changed when I did a walk to see Jackie Kennedy's former Georgetown homes. It got me into the lovely, quiet residential streets (as well as fantasizing which one I'd want to live in if I could ever afford one): http://www.georgetowndc.com/self-guided-kennedy-walking-tour/

And it's great that the Circulator is now free. I use it a lot, as it really fills in gaps in Metro service.

Smithsonian museums. I'll put in a plug for the Hirshorn Museum as it's often less crowded than the other museums due to it containing modern art. Through the fall, there is a hands-on exhibit called Pulse where visitors can put there hands on a machine that measures your pulse and see the resulting pulses in water ripples, electric lights, creating art. It's actually quite fun.

You could go on a WashingtonWalks.com tour that tours a particular neighborhood or parts of city where someone famous lived. I went on the Hamilton walk and learned more about the man than I learned in school.

National Cathedral

@acraven..., I actually haven’t booked our hotel yet, hoping to get to it tmrw. I want to stay near the mall area and use my chase points. I was thinking the Holiday Inn-Capital, but I am open to other suggestions!

Tom,

One hour is not enough to see the Udvar-Hazy and get much out of the visit. Two hours minimum I'd think.

Thank you everyone for your suggestions! I would love to go to Mt. Vernon as I have not been. So much to do there!

I believe Tom is right that SmarTrip doesn't save you money on buses, but it definitely does so on the Metro itself. Edited to add: No, no Metro discount with SmarTrip.

I know diveloonie isn't old enough to qualify, but I'll put this out there for the benefit of others: Those 65 and older get substantial discounts on buses and the Metro (it may be 50%) after converting their SmarTrip cards to the senior variety. That costs nothing but means a trip to a staffed window at Metro Center. There may be other places where it can be done. You must show an official document with your birthdate (and I would guess also a photo, but I'm not sure) at the time of the conversion.

The way I found out about the senior discount was that the driver on one of the 5A (Dulles to L'Enfant Plaza) buses charged me just 3.75 once when I was paying cash because I had done too good a job of stripping my wallet of nonessentials. He didn't ask for proof of age (I must have looked really haggard); I have no idea what the official policy is, for giving the senior discount to people paying cash on a bus.

If you go to Mount Vernon, definitely stop in Old Town Alexandria on the way over via George Washington Parkway. It's a great place to stroll, grab a bite, see some lovely architecture and historical sites and the Torpedo Factory Art Center.
https://www.visitalexandriava.com/old-town-alexandria/

As far as where to stay, I prefer local neighborhoods instead of the Mall area. In DC, I really like Woodley Park and Dupont Circle (although there are many, many others like West End, Georgetown, etc.). If you have enough points, there are some lovely leafy neighborhoods that have tons of restaurants and are really pleasant to walk around in and full of foot traffic (unlike the Mall area after hours except where people are biking or scootering around).

Gee, I always thought Agnes and acraven was the same person?

No, not the same person (I'm nowhere near retirement and can't travel as frequently or extensively). But we get together at our DC Meetup when acraven is not traveling.

As for Udvar Hazy, you really need at least 2-3 hours, if not more. It's in a huge hangar full of interesting stuff. Maybe you can see it on your way back to Dulles? An hour won't do it justice, but at least the $15 parking will be free.

There used to be a discount on the Circulator with a SmartTrip, but now the Circulator is free.

I agree that one needs several hours - minimum- for the Udvar-Hazy Center. However, be aware that the only onsite food option is McDonalds. To get other food, you can leave and reenter the parking lot on the same day without paying again, but of course this takes more time.

I also agree that being in a hotel near the Mall isn't so convenient. It's easy to get to the Mall during the day for sightseeing, but after that it's nicer to be near restaurants and stores, which the Mall is not. I can't help with specific hotels, as I now stay with my sister when I visit. If the hotels in the District are too expensive, also consider the ones in Virginia near a Metro stop (Crystal City, Alexandria, etc.).

Not sure someone didn't say this already, but you have to use a SmarTrip Card on the Metro now, no more paper tickets. You pay $10 when you buy the card - $2 for the card itself and it then comes loaded with $8. You can buy them online which might not be a bad idea to get you started, because there can be lines at the metro stops and at first it can be confusing. Also you have to swipe the card on your way into the station and also when you are leaving the station so don't put it completely away somewhere. Of course there is lots of info online about the Metro system.

I like others's ideas very much - Portrait Gallery (active area with tons of restaurants), Mount Vernon, The Phillips Collection (Dupont Circle). I always love going to the National Zoo (free), and there is a 9/11 Memorial at the Pentagon that is worth seeing.
And if you want to go to Old Town Alexandria there is a free "Trolley" that goes from the King Street metro to the waterfront and back.

If you are coming around Memorial Day, the Metro stations below National Airport in Virginia will be closed until after Labor Day. Just an FYI.

Things I did with relatives a couple weekends ago:

Queens of Egypt exhibit at National Geographic. Thought it was well done.

Harpers Ferry. One of my favorite places in the region but you would need to take the Amtrak train from Union Station of you’re not renting a car.

Woodlawn and Pope Leighey House (lovely). near Mt Vernon. (But if you haven’t been to Mt Vernon, definitely go there!)

Alexandria stations will be closed from the Airport to Huntington this summer for platform work starting in late May. Check the Metro website for details.

Have fun!

There used to be a discount on the Circulator with a SmartTrip, but
now the Circulator is free.

Wow! Glad to hear that! To the OP, the Circulator is very convenient and that it's free makes it even that much better.

Speaking of the Circulator....the free days may be numbered.

https://wtop.com/dc-transit/2019/05/free-dc-circulator-rides-may-be-on-the-chopping-block/

I highly recommend the excellent Trip Hacks DC YouTube channel. There are about 100 short (5-7 minutes) videos that cover every aspect of DC.

Two local chains that I love are zburger and District Taco.

Tortilla Coast is an excellent Mexican restaurant and you may see members of Congress there. If everyone is over 21, the happy hour is a great bargain for DC, especially for Capitol Hill.