I did a loop from Paris through Normandy, Brittany, Loire Valley then back to Paris.
It was September, the weather was nice and crowds weren't bad.
Some tips:
-Cancale really does have great oysters.
-Brittany has Pardons (local food, music, costume festivals) all summer. There may have been one or two going in September but nothing close to me. It might be nice to plan a loop that goes to Brittany first so you can plan to attend a Pardon.
-MSM is nice in September, high tides are often high enough to wash over the road to the island and the crowds are manageable.
-I enjoyed the cliffs around Etretat and Fecamp in Normandy. If you like WWII history, there are pill boxes along the cliff trail that were abandoned and never destroyed. They are mostly full of dirt though. Regardless, the views are spectacular.
-For me, renting a car in Rouen worked well. I trained from Paris early, spent a half-day in the old center, then picked up a rental and drove to my first Airbnb. I dropped the car in Chinon, traded for a bicycle, then trained back to Paris from Amboise.
-Favorite towns to stay in Brittany were Dinan, Vannes and Auray. Also enjoyed visiting St. Malo and eating in Cancale.
-Favorite non-touristy activity was Cairn de Gavrinis. Took a ferry tour from Lamor Baden. It's all in French but takes you to a passage tomb much like Newgrange in Ireland plus a partially submerged henge. Seeing the tide going out there was pretty amazing, almost like river rapids through the islands.
