Trip to Mont St Michelle from Paris

We are doing a day trip to Mont St Michel from Paris. We purchased our train tickets well in advance on Captain train as it allows you to purchase the train ticket and the bus ticket needed at the same time that is needed to get from the train station to Mt St Michael. The Train/bus trip is about 3 1/2 hours...so 7 hours round trip. There was many departing times and return times to choose from. The further out that you buy the tickets to better deal you can get. I was going to do this trip with a tour group but the cost was going to be close to 400 euros for two of us, too rich for my taste. For less than 1/2 of that we are doing it ourselves.

Our journey is Gare-Montparnasse to Rennes
Rennes to Mont-St Michael

Then back out getting into Paris around 9:30pm.

Good Luck!

I read the previous comment on this thread and just was looking for a few more details. If doing a Day trip from Paris to Mont St. Michelle, how much time is allowed for round trip train/bus transit and how much actual time to you have there visiting the area before the return to Paris?

We have another visit on a different trip that has a cruise Port stop in Honfluer, would it make more sense to do a day trip from Honfleur Port to Mont St. Michelle vs from Paris. Just trying to evaluate the logistics of travel time round trip vs. actual time visiting the area. Any input is appreciated. Thanks