We'll be traveling by car on a Thursday in September and are seeking must sees, eats, picnic spots? along this drive. Any suggested routes appreciated!
It depends on your interests, but shortly out of MSM is the Brittany American Cemetery (actually in Normandy). It has half the graves of the more famous American Normandy Cemetery, but one tenth the visitors.
It's effectively a half day drive if you stay on the Autoroutes, and Le Man's is the natural choice of somewhere to stop and visit.
If you're avoiding the Autoroute it's a longish half day drive (about 5 hours), and the obvious stop would be Fougères.
It's a long drive. You may only have time for short stops, many of which are 15 minutes each way, off the autoroute. You might pick the largest place, or something that's really unusual, like Fontevraud Abbey (UNESCO WHS). Will you visit Villandry from Amboise?
They are early in the drive, but both Fougères and Vitré are very nice visits, loaded with history too.
Autoroute or no Autoroute, I'd strongly recommend driving via Fougères and Ernée, then turning south towards Laval (instead of going via Rennes). It is a more pleasant drive, and the divided highway between Ernée and the A81 autoroute near Laval speeds things up.
Fougères and Le Mans are the two stops I had in mind; I would lean towards Fougères because it is a quicker stop. Spending less than ~3-4 hours in Le Mans would be rushed, in my opinion.
If you do not take the Autoroute at Laval, you will most likely drive through Le Lude, which has an interesting castle especially inside (lots of decor) and the gardens. Worth a stop if you pass by. Otherwise do not fall asleep on the long straight stretches between Laval and Tours.
In case you take the route through Le Lude worth is making a detour to the impressive abbey of Saint-Pierre de Solesmes near Sablé-sur-Sarthe. It has still a living community and still celebrate the liturgy in Latin. Together with Le Lude a nice off the beaten path experience, and a good way to split up that long drive between Laval and Tours. Enjoyed both 20 years ago, but wouldn't skip Fougères.