I just loved so much about Brittney and would love to go back. We stayed in Dinard and did day trips to Dinan and St. Malo. Very much enjoyed all those cities. We had a lovely sunny day for St. Malo and we brought a picnic (including a Vouvray from Loire) and had lunch on the city wall. Highly recommend! Dinard felt different (in a good way) to us than other French places we've visited. We liked the retro, beach resort vibe and the coast with views of St. Malo were lovely. Mont St. Michel is on the border of Brittney, and I would think you would have time for that. I think it is day trip distance from Dinan/Dinard, though I would recommend staying the night before your visit, not necessary right on the island but very close so that you can get there at the entrance to the monastery 15 minutes or so prior to opening. Note that means probably an hour early since you need to get from the parking lot and up to the top of the island. We also went over the night before at 5ish, and got some great pictures and had some quiet time then.
We REALLY, and I mean REALLY enjoyed the very small village of Locranon. It is so beautiful with gray stone buildings, blue shutters and flower boxes. We overnighted there. Close is Quimper which is a larger city where they make the famous ceramic and they also have a pretty town center. Also close is Douarnenez with a lovely coast.
In the area, try the savory crepes and the kouign amman pastry.
You don't that I noted say when you will be in Brittney. We were there at the end of September/beginning of October and it was a bit chilly, but that was fine with us. It also meant it was a bit quieter, except for Mont Saint Michel and St. Malo which seem to be always busy.