The Centre Pompidou
Centre Pompidou (locally known as Beaubourg) is my personal favorite museum in Paris, and is also fun for a short visit. This big colorful block in the middle of the city is quite unique, being the largest modern art museum in Europe, with famous works by Matisse, Picasso, Warhol, and others among its displays. It's also home to an interesting library, numerous special exhibits and some active research centers. Of course if you want to visit the whole museum and take your time, it isn’t possible on a short visit, but with 2-3 hours to spare, you can see many of the great works and get a good general overview.