I used to live in Caen, from where it is perfectly doable as a day trip. From Paris, that might be a stretch. The highway from Caen is single-lane in places, and often gets jammed by a slow truck, so driving in from Rennes is probably good advice.
In July and August they do guided meditative night-time visits with music at the Abbey. You have to reserve ahead of time. October is quite likely to be rainy, unfortunately.
I would avoid staying at the tourist traps on the Mont itself. You should look for accommodation in nearby Saint Malo, the walled corsair city (you may be familiar with Quebec City's architecture, which draws from Saint Malo). Of course, that would require renting a car.
The food in Normandy is excellent, if sometimes overly rich. Breton food is good as well, but Brittany was historically much poorer than Normandy. Once again, avoid the tourist traps on the Mont Saint Michel itself, but there are many excellent and very reasonably placed places in Saint Malo, Cancale and many small towns where you wouldn't expect it. The Michelin guide is a good place to plan.