You will probably get a million suggestions about places to go and see. I've rented an apartment for a 2 week period the last three years in Paris, and though I am grandmother-age, I was a teacher and always look through a child's eyes. You are talking about teenagers, however, so it depends on their interests! Children of all ages have traditionally gone to the Luxembourg gardens and rented sailboats to float around the central pond - it is, for sure, simple but quite beautiful and something that would connect them to history and the day, while you just enjoy people watching. Be aware that there is almost no such thing as a picnic, and in most places you can't walk or sit on the grass unless you see the Parisians doing it. However, if you stroll down Rue Mouffetard in the Latin Quarter, you will find all kinds of great shops, a feel of old Paris, and lots of Creperies where you can order a huge crepe at a window and walk down the street eating it, no problem. A word about the Eiffel Tower undertaking: unless you want to spend hours waiting in line, and experience the most crowded place in Paris, I don't think going to the top is worth it. It is tedious. I did do it on my third trip, feeling it must be done, but it was a huge time and patience drain. If they've been in high buildings before, this is not so different. Likewise, never mind trying to see the Mona Lisa - you have to fight to get anywhere near it, and you are fighting very determined tour groups who are all taking pictures but not really looking at it! Going over to Notre Dame at dusk, watching the lights come on, seeing the boats and reflections of light in the Seine is really beautiful and memorable. If they are into books, go to Shakespeare and Company bookstore on the Seine, and if they are artists, go into Sennelier very closeby where Picasso bought his art supplies. Those are my highlights!
