I live there. I rinse the fashion expos. Dior is one of the best, imho. You could then go to the takeout counter at Ladurée on the Champs-Elysées, and have your coffee and macaron at the seats there, rather than the overpriced seating.
If still on, Louvre Couture is amazing. Usual Louvre ticket, but head to Richelieu wing. Haute-couture dresses interspersed amidst beautiful things.
Louis Vuitton’s LV Dreams expo is free and good for teens, as not too long. There’s a cafe serving expensive cakes with LV logo, with fab view of the Seine. Much cooler than Angelina. It’s directly opposite La Samaritaine, which is a lovely department store. Book your slot here https://lvdream.seetickets.com/timeslot/lv-dream
Another free place is Le Petit Palais. Great pit stop on the CE, with decent loos and cafe. Current (paid) expo is Worth. Free art and sculpture museum that’s not too long.
Other fashion museums abound, depending on taste. The YSL is not as interesting as Dior, but still ok. Then there’s the Alaia gallery. The Paris fashion museum is good: https://www.palaisgalliera.paris.fr - just check out what their current expo is.
Fragonard Musée is free, pretty, and kills 30 minutes. There are mini workshops you can do to create your perfume. It ends in their store, but this is crazy full of tourists. You’ll get a better browsing and service experience just round the corner at their flagship store on Rue Boudrée, just off Auber RER.
Food-wise, the Pink Mamma chain as mentioned above is great. You need to book or be prepared to queue. My top tip for teens would be La Felicità; it’s part of said Pink Mamma chain, but is in an enormous industrial space. Think huge food court. The loos are very, very instagrammable.
Free viewpoint is the Pompidou Centre. This is closing in September, but you might be able to access the viewing gallery still (no charge to get up there).
If your teen can bear some culture, the Fondation Louis Vuitton always has great expos, plus you can then exit into the Jardin des Acclimatation for free. Gorgeous gardens, some fairground rides, mini zoo and aviary, lovely Korean gardens with deckchairs. Very peaceful spot for a picnic.
Or, the current expo at Musée Jacques Branly (next to Tour Eiffel) is called ‘Fil d’Or’. It’s amazing… so good, I’m going again. It traces the history of gold, but with a special emphasis on haute couture. The golden dresses and costumes are just stunning. The museum restaurant is very nice (in a chic, Parisien ladies who lunch way), and the gardens have the best view of the Tour Eiffel for that photo op.