One day to get a flavor of Paris

I would avoid the HOHO bus - it’s kind of the antithesis of just “being” in Paris. There are city buses that go by major sites: I will paste in the details at the bottom of this reply. This option is more flexible, IMHO, and much cheaper. He’ll also be seated next to locals, not penned in with other tourists.

Your friend is young, and presumably fit enough to spend time just walking around. That is what I would suggest. There’s nothing like just wandering through little streets and soaking in the sights and sounds.

One approach would be for you both to identify cool places he would like to see and then construct a walking itinerary for him that takes him by those places. I’d encourage him to blend the walk with a stop or two for wine or coffee/pastries at a cafe (the less touristy the better) to soak up the local ambiance. If there’s a local market somewhere in his day in town, that might also be swell.

Edit: Mardee has some great ideas for cafes and food!

Edit: Seine cruise? Why? Again, you are crammed in with tourists and not actually “being in Paris.” How about a walk along the Seine with a picnic? Bottle of wine, sandwich jambon, tarte au fraises… and a clear view of the Eiffel Tower when it lights up? Dang. Gorgeous. (Apologies for seeming crabby about tourist hacks like HOHO buses and the like… but Paris is so wonderful and easily accessible by foot that I feel corrupting the magic with crass tourist shortcuts is not an ideal option.)

Rick Steves has walking plans for Paris, as well. Full disclosure: I have not followed one, but I’ll bet there are quite a few people on this board who have, and loved them.

Bus details: The Bus 69 is known for passing by major sights like the Eiffel Tower, Les Invalides, Musée d'Orsay, and the Louvre. The Bus 42 also offers a scenic route, including the Champs-Élysées, Place de la Concorde, and the Opéra Garnier. The Bus 89 is great for exploring the Left Bank, with stops near the Jardin du Luxembourg and Panthéon.

This would be my rough outline for an epic walking tour of the highlights of central Paris, avoiding the major thoroughfares:

Part 1

Part 2

Take a boat ride. On the Seine, Vedettes de Paris or Vedettes de Pont Neuf.

I would eat and drink my way through Paris, maybe walking through Montmartre, St. Germain, and the Ile de la City; stopping at patisseries, boulangeries, cafes and so on. And I also vote for a Seine cruise; the ones in the evening are really nice. We took the 8:15 cruise from the Pont Neuf, which got us to the Eiffel Tower just in time for the sparkles. The river is beautiful in the evening with all the lights.

if he's going to Notre Dame, tell him to stop at Berthillon to get some ice cream, which is near there.

Balso: those are nice routes!

I would eat and drink my way through Paris, maybe walking through Montmartre, St. Germain, and the Ile de la City; stopping at patisseries, boulangeries, cafes and so on. And I also vote for a Seine cruise; the ones in the evening are really nice. We took the 8:15 cruise from the Pont Neuf, which got us to the Eiffel Tower just in time for the sparkles. The river is beautiful in the evening with all the lights.

I second this, and I like the idea of the evening Seine river cruise, but this time of year it is light out until 10 PM, so I would catch a later boat to be dazzled by the Eiffel Tower light show...

Rick Steves recommends taking Bus #69 which starts at the Eiffel Tower and ends near Père Lachaise Cemetery. Sit near a window and take in many of the sights that tourists want to see in Paris. RS says that it's much better than the HOHO and costs much less. Plus, you'll be amongst local Parisiens. Here is the route:
https://www.ratp.fr/en/plans-lignes/busratp/69

if he does the 69 it is imperative to get on at the start of the route because it fills up and then you are standing seeing nothing. AND he would need to go clear to the back or otherwise seek out an awkward higher seat because buses cater to those who cannot use the metro and a healthy young man would need to give up his seat. I am an ancient crone and I have to give up my seat to people who get on with canes, crutches and walkers.

What is his dream of Paris? Do that. There are lots of curated walking routes -- books, cards, on line walking maps and if it appeals to him choosing a self guided walk in the Latin Quarter or Marais and Iles would give him a flavor of Paris.
Maybe do Notre Dame and then a walking tour of the islands and then the Latin Quarter. Vedettes Pont Neuf is central and would be a nice boat tour. We enjoy walking along the Seine on either side, from Mitterand Library or so up to the islands or from near Notre Dame up the right bank. It is pedestrian and bike zone, there are lots of bars and the occasional toilet available and just really quite lovely. He could walk up to the Tuilleries and get off there and see the outside of the Louvre, get lunch or watch the kids sailing boats on the fountain pool.

An important thing is a place to hang out in the evening. There are lots of options. Franchette, the boat bar by the Eiffel Tower has great cocktails, awful bar food and a fabulous view of the Tower at night; not sure about the youth vibe although when we were there there were a number of young couples. There are bars with music and a younger vibe -- but I am not able to recommend specific ones -- maybe someone hip here could.

I absolutely agree that HOHOs are a great way to waste the day and see what is easy to drive by on a bus -- and the waits are endless. For an able bodied person they are a time suck with little benefit. And IMHO it is important to not try to cram in to much wander and hang out and have coffee in the afternoon and cocktails and music in the evening seems like the right idea.

Second the Rick Steve Walks (in the book). Do the Historic Paris walk on arrival evening and then the Champs-Elysees walk (or the Left Bank Walk). The evening boat ride would be a great addition, see the Eiffel Tower Twinkle! These would give your young friend a good taste of "seeing" Paris and also entering Notre Dame.

Rick Steves recommends taking Bus #69 which starts at the Eiffel Tower and ends near Père Lachaise Cemetery.

With the far less than 24 hours that your friend has in Paris, I absolutely would not recommend the 69 bus.

He needs to stay very central in the general area between Eiffel Tower and Notre Dame, and a boat ride is a perfect suggestion in addition to the walking.

Beauvais airport takes a hell of a long time to get to/from, so his time here is even shorter than he thinks.

Kim is right. He is looking at a long ride to Paris from Beauvais -- you can have a great dip into a new town in a short afternoon/evening and morning but it depends on figuring out what you most want to do and not trying to do so much more that it is all about logistics.

He can easily do Notre Dame first thing in the morning when it opens--8am. There is no line and hopefully he will be able to enjoy the cathedral before it starts getting packed.

Sights mean different things to different people. He can make a list and then buy a metro ticket for the day, 12 euros each day, and then he can plot his adventures using the efficient and fast metro system to get himself around from place to place. I suggest that he download CityMapper App as well as RATP app (for the metro- and he can even buy the metro tickets and load them into his phone-- I used my iWatch-super handy). He can use them ahead of time to help him map out his adventures.

I stayed in the Bastille area, Marriott has a Moxy there that was very convenient.

I'm not familiar with the airport, but if it is served by the RER--that would probably be the quickest way to get in and out of town. Good for him to create a fun little sidetrip for himself.

Thanks for all the pointers. This will really help me help him. I haven't been to Paris in over 25 years.

He's 29; everything that sounds exhausting and inconvenient to those of us in an upper age bracket, won't be for your friend! It's not ideal, but hopefully his first taste of Paris will inspire him to come back for more!

Regarding bus 69: bus speeds have become abysmally slow in the past 2-3 years. Brisk walking is often just as fast, and you see more! I no longer recommend Paris city buses to people with normal mobility, with the exception of some faster corridors (Rue de Rivoli, Boulevard de Sébastopol).