did this tour a few years ago and it was great! Great food, a mix of interesting chateau and towns, the canoe trip, the caves with prehistoric paintings, Pont du Gard, and much more.
However there are a few things I wish I had planned out before the trip. RS tours don't rush you around constantly but give you chunks of free time. I think it's a good idea to plan something the Sarlat market day. It is a great market but even walking slowly and looking at everything (in France look, don't touch!) I still had most of the day left. In retrospect I wish I had either planned for a train trip out of town or to rent a car to visit another one of the prehistoric caves in the area.
Another day to make afternoon plans for is the very first full day of the tour. All the Loire tours start with Sunday meetup and dinner, probably so they end up in Sarlat on market day. The problem is many smaller French towns are closed up on Monday and Chartres is no exception. You'll do the Cathedral tour Monday morning and then it's on your own, but most of the shops and many restaurants will be closed. If I was doing it again I'd look up in advance train schedules to nearby cities such as Le Mans, or perhaps the bus schedule to Orleans and plan a daytrip