Mont St Michel

You may find many things closed during that time. We had that experience in Bayeux in Nov. 2019. About half of the shops and restaurants were closed until early December. In one village near Utah Beach we couldn’t find a single restaurant open. We managed to find a bakery that served ham & cheese croissants and had a picnic in the car. The owner said everything closes at that time of year. Just be aware that your options may be limited.

o2bnfr, I see that you have just joined this board. If you look at some of the voluminous past posts (Search box top center) you will see that some posters (like me) feel that a few hours at MSM are enough. So IF the place is open, why not go?

But there's a hint in your OP that you are seeking a larger, "local life" experience in the area. Now, because the distant approach to MSM includes such things as an Alligator Farm (!), and other touristy "strip" experiences, I question that plan. And in high and shoulder season, it can be convenient to traverse (on foot I mean) the purely local strip of off-island hotels, snack bars, and supermarket that you pass between the vast parking lots and the short causeway to the island itself.

Off-season, the hint that you need public transportation (?) from Rennes to MSM may be the major research point you need. We had a car, in high season, so I'm no help there.

Mont St.Michel is open year-round with the exception of 3 days. There are buses traveling from Rennes’ train station to Mt. St. Michel in 70 minutes. Alternatively, you can take “BlaBla Car,” a shared-ride company similar to Uber.