Starting my research, have a few questions.

Barcelona and Madrid are easy in and outs and easy cities for day trips. May is still going to be warm in Barcelona and getting to be down right hot in Andalusia (which you don't mention). I use Wunderground for historical monthly temperatures..the actual temperatures per day rather than the average that tourism websites give you.

You could consider a couple of days on the coast from Barcelona then train to Madrid for visit and return flight home.
Alternatively, for something lesser traveled, try Zaragosa which is on the train route between Barcelona and Madrid.

1. May would be nice most everywhere. The south has the potential to be very hot, but in my experience it has been plesantly warm - 80s, not sweltering like in high summer.

2. Barcelona is a good choice. Madrid is also interesting, and I personally am interested most in the south. So Barcelona and Seville would be my top suggestions. Seville is lively and has excellent food.

3. I don't have specific hotels - would the 4 of you be in one room or two? If you want 2 (or even 3!) rooms then an apartment would definitely be more economical. Rooms for 4 in hotels exist but are harder to find - many hotels will suggest you reserve 2 rooms. I have younger kids and always get an apartment when traveling with them because of the privacy. With adult children, 2 hotel rooms is also an option.

4. Barcelona experts will jump in here but I'd look a touch north in Eixample, or in the Gothic quarter, or El Born. Being near a metro is important.

5. In Barcelona the beach is very easy to get to - just hop on the metro. The Barceloneta beach is right there and is nice. I hear there are quieter beaches further out (still on the metro) or in outlying towns (train). I can't speak to that, I am not a huge beach person and Barceloneta was enough for me. There is no beach in Seville or Madrid, but there are plenty of beach towns that you could go to if beaches are a priority - just pick an area of the country and suggestions will roll in!

BARCELONA

1.) Any month is good as there are plenty of things going on all year round, yet if you want beach, Apr-May and Sept-Oct are, IMHO better than the summit of summer because the weather is less hot and they're less crowded. As an example, this was today in Platja del Bogatell, one of the 10 urban beaches in Barcelona. For reference, the Mediterranean is a warm sea -in comparison with, say, the Atlantic or the Pacific- and water temperature in mid-Sep would be around 70F/21ºC and around 60F/15ºC in mid-May in an average year. The hottest moment in the year for sea temperature tends to be early Sep when it reaches an average of 74F/23ºC. Other nearby great places for those seeking beach are the quirky town of Sitges, 45' south of Barcelona by train and also Tossa de Mar, just an hour and a half from Barcelona by bus, among many other.

2.) Suggested escapades from Barcelona

3.) ---

4.) Busy, noisy, flooded with tourists. IMHO.... these are better options.

5.) No, wherever you're in the city, the urban beaches are just a short metro or bus ride away... 45' tops from the most northern districts, less than 20' from say, near Plaça Catalunya. Or a short walk if you're in one of the southern neighbourhoods. Get your bearings here.

Wow! Thanks everyone, lots of ideas and places to research. I did find a hotel in Madrid that had connecting rooms for a decent enough price, not cheap but I've spent more. I've never rented an apartment before, I'm a bit intimidated by that, but I should at least look into it.

Apartments... that's a very personal decision of course... but when I travel and have to stay somewhere for more than a couple of nights I tend to prefer them. For one, they're bigger than a hotel room and despite you'll probably end up paying a similar price in many cases, the freedom to go in and out and the spaciousness is always worth. Also, having a kitchen/kitchenette also allows you the freedom to cook at your leisure, whenever and whatever you want.

If you go down that route, check bookings.com, homeaway.com or airbnb.com to start exploring the offer... there are other portals, but these are the most popular. Just one thing to remember: always check whatever apartment you fall in love with for the HUTB number somewhere in its advertisement. The HUTB number is a license number that ensures you're renting an apartment that complies with the local law.

Thanks Enric. Also as far as beaches in Barcelona, I read it's just a 4 min. Las Palmas, is that correct?

To be clear, HUTB # is for Barcelona. Other cities may have a different abbreviation. For example, Sevilla is VFT/SE/####. Many cities have not yet determined their short let registration process.

Sorry Lulu, I don't get what you mean... there's no Las Palmas beach, or metro station, or anything.

Sorry, autocorrect. I meant that I read the beach is only a 4 minute taxi ride from Las Ramblas.

Ditto the recommendation for an apartment. with a family, you want room to spread out. We just did it in Spain with 2 couples; so much nicer.

In Barcelona, we stayed in Eixample area and rented from habitat apartments. While in Barcelona we did a street art tour, which was one of the highlights of the trip.

In Madrid we stayed on Plaza Santa Ana and rented from Spain Select.

We went to Seville and I think it is a great city. Lots to do, great food, very lively. There we rented the apartment from homeaway.

We also went to Palma for a few days. If you want the beach, you may consider one of the islands. The airfare to and from is quite inexpensive.

Lulu, that'll depend... firstly, Les Rambles is a long nearly 1.5 miles boulevard; secondly, there's no beach at the bottom of Les Rambles but a marina, beaches are to the right. Please check the map I sent before.

Book La Ribera or Barceloneta for beach access.

Another thing that is important to us is being able to walk to dinner. I know I can do that in the Las Ramblas area or Grotto Quarter. Am I able to do that in the Eiample area? I do see several apartments there on airbnb. We enjoy being in the center of things rather than nice and quiet.

Also, I'm leaning towards Barcelona and Seville. It sounds like I could fly into Barcelona, stay 5 nights and then catch a fast train to Seville. Should take about 6 hours and we like train travel. Good experience for boys, too. How many nights in Seville? Would 4 be sufficient...too much? I know that is subjective. For example when we were in Venice, 2 to 3 nights was sufficient.

Travelled with our sons to Barcelona when they were 18 & 19 and then again when they were 24 & 25. They had a great time! Beach was not part of our plan as we live in SoCal and can go daily at home. If beach and surfing are high on your agenda, I would suggest considering San Sebastian too.

Sevilla is fantastic too. Similar to Barcelona in its numerous activities but unique too.

In Barcelona we stayed in apartments in Gracia and Eixample near the L5 metro Sant Pau stop. Good food nearby and a quick walk or metro ride anywhere we wish to go.

In Sevilla, the Santa Cruz area is very nice. Saw many nice Hostals and Hotels. We got an AirBNB near Plaza Alfalfa that was excellent.

There are restaurants to walk to all over central Barcelona. There will be excellent recommended restaurants and simple cafes in eixample, gracia (north of eixample), gothic q, ramblas, el born, raval and more. So you are set there - but if you are seeking a particular special restaurant you might want to see where it is and stay nearby if walking is that important.

Metro and cabs are easy if you do want to go somewhere farther out. Also you can easily walk from one area to another. As an example I put in my last apartment there (near Tetuan metro, eixample) walking to the Barcelona cathedral - as a marker of the old town. 1.5km, under a mile. Did that walk many times, so staying in one area and walking to the other is easy.

As for the beach, I just used google maps and put Boqueria Market on Las Ramblas to Barceloneta beach. It's showing 15 minutes by cab, same by metro, and 30 minutes walking.

Seville - 4 nights is good. If you wanted you could do a day trip (45 min by train) to Córdoba for one day. But there is more than enough in Seville for 4 nights. Barrio Santa Cruz is the old quarter, lots of cafes and restaurants. I would definitely stay in that area - it's just the most lively.

So much to think about! It looks like I might highly consider an apartment when I look at hotel rates. It's really not bad until you start booking 2 rooms. Although we have traveled many times in the same room (2 queen beds), those aren't as numerous to find and DH is getting tired of all of us in the same room. Renting apartments is a bit out of my comfort zone, but so many people do it. I will make sure that it has the official code...I have seen that listed on Airbnb.

If we do Barcelona and Sevilla ( with day trips), we will fly round trip from Barcelona. Take the fast train from Barcelona to Sevilla. Not sure how to get back to Barcelona. We could train back or fly back the day before departure. I tried doing open jaw through Delta, and it said it didn't have a sister airline when I tried going into Barcelona and then fly back from Madrid (we could train to Madrid, shorter distance).

Now that I'm sure the cities I want to go to, I need to figure out logistics. I think 5 nights Barcelona and 4 nights Sevilla and 1 night city of departure is an option.

I know I'm all over the place, so my apologies for that....but I am a multi tanker and taking to heart everything you are saying.

Airbnb is good, also look on booking.com. That's where I find a lot of the apartments I use.

Also you can google "aparthotels in (city)". Aparthotels are just that - hotels with multiple apartments they rent instead of regular rooms. Might be more in your comfor zone, but they can cost more than an apartment from Airbnb or booking.com

For something simple ...
5 nights in Barcelona, with a day trip to Montserrat or Girona.
5 nights in Seville, with day trips to Cordoba, and Jerez or Cadiz.