We are going on the RS trip that begins in Chartres. We have been there before. Of course, we will be anxious to see the cathedral again, which is included in the tour, but there is also some free time built in. I'm wondering if there are any short excursions from Chartres that are worth considering. Bus or train would be fine. I've looked at the Rick Steves France guidebook, but it doesn't really give any information about what might be doable from Chartres. Ditto online searches or other guide books. that city. They all only give instructions on going to Chartres from Paris. Any tips?
There's the 14th century Chateau at Rambouillet that looks pretty good. Just up the tracks a ways toward Paris. There's another nearby at Maintenon.
I suggested looking for day trips in my review of the RS Loire Valley trip so this is something dear to my heart! https://community.ricksteves.com/travel-forum/tours/loire-to-the-south-of-france-trip-report
Paris is about an hour train ride from Chartes, so if I was doing it again I'd open a map and figure that anything in about that radius on a train line is doable. Find the name of the department that Chartres is in and the neighboring departments, then look for tourist attractions in them. Or just find places that might be interesting to visit, places where every other tourist clutching the blue book hasn't been. Using the DB train site it looks like Le Mans is about 1 hour away, so is Châteaudun in a different direction, and Nogent-le-Rotrou is even closer and seems to have a nice castle. This is just from looking at google maps and the DB site, I've never been to any of these. I think you can pick just about any town within an hour or so, ride out there, see the main sight(s), have a bite to eat, spend an enjoyable 1/2 day doing so.
Château de Rambouillet is worth a visit to my opinion too, as also Château d´Anet, some 50km north of Chartres, however no idea about the bus connection. As you are interested in the Loire Valley trip Chenonceau´s Diane de Poitiers spended the last part of her life in the latter and is also buried there. Visited both, Maintenon still on my wish list for a visit.
I vote 100% for the Château de Maintenon.
But there's quite a few things to see in CHARTRES. The stained glass museum is nice, the Beaux Arts museum is fabulous, Maison Picassiette is worth a gander.
But I whole heartedly suggest the Château at either Maintenon or a totally different style of chateau (from the days of building protective castles) in the city of Nogent-le-Rotrou (on the Paris-CHARTRES-Le Mans TER train line). I'm glad to suggest any restaurants or anything else.
I would also ask your RS guide for ideas. The free time will be in the afternoon of Day 2, I think, but you can talk with them at or after your group dinner on Day 1.