If the skies are blue, just walk down by the Seine, make sure to cross a couple of bridges along the way, look around, and take a bunch of pictures. Paris is a magically beautiful city if you like architecture. Just be sure to bundle up, because it's pretty windy down by the river.
As far as museums, if you like impressionist art (Van Gogh, Monet, Manet), nothing beats Musee d'Orsay. On the bottom floor in the back, there's a scale model of the Opera Garnier dissected in half, so you can see what's going on inside. Very cool. Also, go to the cafe at the top and go outside onto the balcony for another really nice view of Paris.
You could do both Rodin and Orsay in a day. Rodin isn't that big, and Orsay only takes a few of hours itself and they're not that far away from each other.
