Couple of ideas:
If the 18 year old (or you) is even vaguely interested in technology/science, then this is a really great Museum that is often overlooked:
https://www.arts-et-metiers.net/musee/visitor-information
Near the end of the tour is Foucault's Pendulum, which is amazing to see. More on that here:
https://en.wikipedia.org/wiki/Foucault_pendulum
For a really amazing selection of steaks and other dishes (go hungry):
https://beefbar.com/paris/
The have a "steak frites" with a "relais" sauce which is arguably better than L'Entrecote.
The Louis Vuitton foundation currently has an unbelievably comprehensive Rothko exhibit. It's extremely unlikely that there will ever be a time when this many Rothko's are all in one place ever again. And the building itself (Louis Vuitton) is extraordinary:
https://www.fondationlouisvuitton.fr/en/events/mark-rothko