As far as what to see, I agree the Louvre is a waste if you're talking
part of a day. Ditto the Musee d'Orsay (less extensive, but lines). I
enjoyed the Pompidou, but if you don't like modern art...
Picasso Museum is interesting, but no real top works.

Besides art, you can go up by Sacre Coeur in Montmarte and look at
city. Some nice outdoor music in the big parks.
My favorite thing is just walking around.