The Louvre

Of course, you have to go to the Louvre. It's the most visited museum in the world. It has collections spanning the entire history of art (up to the impressionist and modern era which are in the Musee d'Orsay and Pompidou). There are extensive Egyptian, Greek, Roman, as well as a huge selection of Babylonian art. But we had to skip all that.
We went most specifically to see the Italian painters; Raphael, Caravagio, Titian, and of course Leonardo Da Vinci, who spent the last years of his life in the court of King Francis I. This explains why there are more paintings by Da Vinci here than anywhere. It is incredible to look at each one and see the thread of genius that runs through his work. The whole Italian collection is fantastic, but what do you expect, it is the Louvre.
The Louvre is so big that you'd probably need a month to see everything. Rory and Jasper reached their limit after the Italian paintings, so we went to one of the several cafes inside the museum and took a break. We then went to the German and Dutch sections which, to me, were not as impressive.
Rory and Jasper each had the camera for some time, and when I went to look at the photos they took, I had a bunch of pictures of ladies in finery and a bunch of pictures of cats, dogs, horses etc...