Well, there are all those classics, famous, very impressive over rated ones (that other users will mention i'm sure) but when I went this past summer, I had the most fun at down played cafes and bars. That's where the REAL Paris is.
...however I do encourage musuems. One always learns at least one thing and the French really know how to put on a good display. Especially the Lourve.
Amuse-toi!
(haha I just read the other users' comments. Yeah, beware of the airport, especially Charles De Gaulle--it's terrible! but you can escape the ..uh..not-so-fun-French but being in places tourists tend not to hang out at so much as I recommended