Mused d'Orsay is my favorite art museum anywhere.
Charles de Gaul airport to downtown is pretty easy and quick by train. The Metro is easy, and Paris is a great city just to walk around.
Yes. But buying that ticket at CDG can be a pain -- it took us at least 45 minutes so be prepared. I second Jesselp's comment about walking. I would spend a good part of this layover walking from the Louve to the Invalides with a stop at the tower. Then I'd find a really good restaurant to stop at for lunch or to end the day.
One of my favorite meals was a cassolet at a place called "Chez Eux" near the Invalides. (
http://www.chezeux.com/). There are more than a few good restaurants in the area or along the way (more or less). These can be pricey by US standards but well worth it.