I'll second the suggestion for the Musée Marmottan Monet; it's filled with wonderful Monet artworks and well worth the visit.
If you're a Monet enthusiast as I am (he's my favorite artist), a day trip to Giverny to his home and gardens is worth the visit. The grounds are really amazing, and standing at the end of the lily pond, where you can envision his creating one of his great lily pond artworks, is a wonderful experience.
Other places to visit:
My favorite chapel of all time has to be St. Chappelle; after you go there, you can walk over to the Latin Quarter and wander around, find some great places for lunch. Some of the finest stained glass you'll ever see, on a par with Chartres or Notre Dame de Paris (which I can't wait to revisit in a couple of years).
Montmartre is worth a visit as well; it's known as an art district and has a lot to offer.
We love the Rodin Museum; it's a peaceful visit since most of the big sculptures are outside.
Dinner: if you're on the Left Bank, look for Restaurant Allard; it's been around for many years and is a great place to eat (Hemingway used to eat there back in the day).