oh my... so much to add. I promise to keep it simple.
You MUST see Sainte Chappalle. If you can get to one of the afternoon chamber concerts you should try to go. It is simply exquisite!
The Musee d'Orsay is a MUST. If you are limited for time, start on the top floor where the Impressionist and post-Impressionist collections are housed. The museum, itself, is fascinating. The interior of the 1st Parisian train station to have electric power was designed by Gae Aulenti. The art collection is primarily from the former Musee Jeu de Paume.
You should also see the Musee Rodin and the Picasso museum. If you are in a museum mood... I love the sculpture garden of the Rodin. (The Thinker, The Kiss...)
A little-known, but fascinating place to visit is the Musée Jacquemart-André. It is a magnificent mansion in the heart of Paris built to bring together one of the most important private collections of paintings, and offers a glimpse of how Parisian society in the 19th century lived.
There are SO many restaurants to recommend, but one of my favorites in the world is Restaurant 404. It will transport you to Morocco. The food is delicious and the decor is rich. The address is 69, rue des Gravilliers. You need a reservation.
I also love the Cafe Voltaire, 12 Rue de l'Odéon. It's small, and generally filled with Parisians. The lamb is extraordinary.
Oh...there are SO, SO many suggestions I could give you.
What activities are you interested in? (I should have asked you first!) Shopping? Srolling? Chocolate? Let me know!
"Two drifters off to see the world. There's such a lot of world to see. . ."