My son will turn 21 while we are visiting Paris in July. It will be a Saturday night. I'm looking for some special activities, places to dine or nightlife that would make the day/night especially memorable. We are a family of four with the second son nearly 18. My sons are on the quiet side and do not speak French. I'd love your suggestions and ideas! Thank you in advance.
Try Fish. Excellent meal, fun bar atmosphere, and the waitstaff can all translate the menu.
Maybe follow dinner with a night cruise along the Seine in one of those bateaux mouches tour boats (or you can have a dinner cruise on a boat instead). The dinner cruise might be expensive but just the cruise/tour boat along the Seine is fairly cheap and an amazing experience, passing under all the lit-up bridges and seeing the old buildings lit up is pretty cool. Paris is pretty safe late at night, I spent my first night there this past summer walking up and down the Seine and around the Eiffel Tower at 1-2AM because I couldn't sleep LOL. Maybe go to the Tower and walk under it? (You can also go up it at night, I believe as some of the stairs/elevators are open late I think).
If you are not averse to bars, there's a terrific Scottish pub just near the Pont Neuf (http://www.the-highlander.fr/) where the atomsphere is really friendly, the beer is good, and there's a DJ downstairs on Friday and Saturday nights.
My husband and I are late 40s, but we love it here upstairs, chatting with the bartender and the regulars - and yet the atmosphere is plenty youthful for folks in the 18 to 21-year-old range. And as your younger sons don't speak French, this might be a nice way to "split the difference." You're in Paris, but English is spoken, since it's a Scottish bar!
If you want a fine dining experience, you could have a very special dinner at a place like Jacque Cagna, which has a Michelin star. It won't matter that your sons don't speak French. A more modern, less formal place, is Ze Kitchen Galerie. Actually, come to think of it, people that age might prefer this place.
A nighttime boat ride is beautiful, and will afford magnificent views of sights like the Eiffel Tower all lit up.
Another idea (which you could do in combination with either of the other two activities) is a visit to a wine bar, if your sons (or you) like wine. A quiet, adult activity. We like l'Ecluse, at 15, Quai des Grands Augustins, in St. Germain just off the river. (Remember that both of your sons are already of legal age to drink in France.)