Mmmm .... I don't know about less tourists but a couple of other excellent day trips are:
1) The Cathedral at Chartre. It's an hour by train from Montparnasse station to the south of Paris and has some of the best medieval stained glass in the world.
2) If you like impressionist art and have seen Monet's water lillies then head to his house at Giverny, 80 km West from Paris. From memory there are only a couple of trains a day but it is possible to do it as a day trip. http://giverny.org/ over 10 years ago