Easy to find, just follow the tower of the Cathedral and the historic preservation area is right around it. It's very picturesque and pretty, lovely to walk around and there are many shops selling pictures of the local lanscape around.
We loved wandering around historic Chartres. It is simply stunning (although a huge central bit near the town hall seems to be undergoing massive building work). We were there during Chartres en Lumiere so there were huge projectors set up for the light shows. It is very romantic and pretty.
Chartres is an old French town. And like a lot of towns in France it's got some cute buildings and river and a load of history.
It's a nice place, but I don't think its unfair to say that you won't be recalling the time you went to the town of Chartres based purely on the charm of this town...
Take a walk , then visit the cathedral in a long walk along the streets of Old Town, without a purpose and see old buildings and new buildings , all cozy and well-groomed . Make sure you eat because the terraces are closing rather quickly and after 20 hours will be a little hard to find restaurant with terrace.