as others have mentioned, the Bateaux Mouche riverboat tours are a nice way to see the city especially at night
I love the old Opera House, if you're into music or theater it's a great stop.
Louvre is huge but the only place to see the Mona Lisa if that's high on your list. If you're just into art in general, I prefer the Musee d"Orsay where Whistler's Mother and some other famous art work is located.