My favorite thing in all of France is the Musee Rodin. That's my journal entry from my last trip there in 2007.
As you can see: between the tea roses, the giant fireplaces, the french doors and the gray velvet chairs, it's exactly what I would want in an enormous Parisian mansion of my own. It's just perfect. Oh, and there's some pretty amazing sculpture there too.
On any visit to Paris, I recommend spending a whole day there, complete with a big picnic in the gardens. And you're just a few blocks from the Musee d'Orsay if you can manage to tear yourself away.