I am staying in Paris. I would like to do a day trip to Chartres. Any ideas?
Chartres is easy. As our friend in the post above said, go to the Gare Montparnasse. There are several trains a day. There's one titled "Paris - Chartres - Nogent-le-Rotrou - Le Mans
and "Paris - Chartres" or "Paris - Nogent-le-Rotrou." One of those will definitely take you to Chartres. I lived in Chartres for a while, so if you have any questions don't be afraid to ask.
My two lasting memories of a day trip to Chartres are Malcolm Miller's tour and climbing the tower - great views. I always take binoculars for the stained glass windows in the cathedrals.