Since you're always tired when you arrive, and its hard to get much done, my suggestion would be to go directly to the TGV (it's at the airport) and go to the south of France. It's about 2 1/2 hours, and you don't usually get that much done on your arrival day, anyway. Spend a couple of days in Avignon or Aix, or go all the way to a Mediterranean town. When you come back, you can take the TGV into an in town train station, so you never have to deal with the trip from the airport to town.

The usual day trips from Paris are Versailles and Giverny (opens 4/1) if you really don't want to move around. I agree, you could spend more than a week just with Paris.