A day of biking to the Chateaux in the Loire valley? It's just an hour by train from Paris and was one of my fondest memories from my trip with my French class. It is really beautiful pastoral countryside and there aren't too many tourists milling around the Chateaux which are really incredible, if my junior high memory serves me I think Chambord and Chenonceaux were my favorite.
Here's a link with some info (though we wouldn't go with a tour company, just rent bikes from a shop)
http://www.bikerentalsplus.com/index.php?id=74&tid=1481#itinerary
