One place to visit (and stay overnight) between Rennes and Charles de Gaulle Airport

Chartres would be perfect. Right off the highway to Paris, and impressive sights (esp the Cathedral).

i would second Chartres, it definitely historic and has a lot to see, plus it is only 70 miles from CDG. In the area you will be touring, I would suggest one place that is somewhat out of the way but worthy of a stop. The American Military Cemetery at Saint James is not well known but it is none the less the final resting place of 4400 of our GIs. The day we visited we were three cars in the visitor parking lot. One was a young Vietnamese-American who we talked to for a while . The other car belonged to an older French couple who I saw standing at the far end of the cemetery. Before we left, I went to where they had been standing and found two graves with fresh potted flowers. Both Graves bore the same marking: Here Lies In Honored Glory A Comrade In Arms Known But To God. It was a touching reminder that they have not forgotten and neither should we.

Rennes has a very picturesque historic district.

Chartres would be an excellent choice if you want to see the cathedral and medieval center. Another good choice, if it interests you more, would be Versailles. You could even make a long day, stop in Chartres and go on to Versailles for the night. But you'd want to visit the palace late that day instead of the next morning, to be sure of making your afternoon flight. Chartres Cathedral and the Palace of Versailles might be too much for one day -- makes me tired just thinking about it. ;-)

Either way, you'll find yourself on the Peripherique for part of your next day's drive to CDG, so allow plenty of time.

Thank you very much to all suggestions. Chartres - it is . Very good idea to take a boat tour of the Golfe de Morbihan, only my husband and I we both easily getting seasick . How big is waves there?