We will be in Le Mans and want an easy and fast way to get to Mont Ste Michel, hopefully in one day. Then going back to Paris. The trains seem to only have 1 train per day, leaving Mont Michel. Is a car rental the best and most efficient way? Thanks for any advice you can give.
Most efficient is train from le Mans to Rennes (1hr), rental car or bus to Mt St Michel, back to Rennes the same way, and train to Paris (1.5hr).
You can check schedules for both trains and SNCF-operated buses on the SNCF website. There are three morning buses from Rennes to MSM in February. I don't know whether they will be more frequent later in the year.
There are four afternoon bus departures that connect with Paris-bound trains in either Rennes or Dol de Bretagne.
A rental car will give you more time flexibility, but by the time you deal with the rental-car agency twice, I'm not sure you'll come out ahead. However, there are some nice towns you could visit in the area south of MSM. I liked both Vitre and Fougeres (though you don't need to have a car to see them).
Oops. I'm sorry. I had Rennes rather than Le Mans stuck in my head. There are four morning trains from Le Mans that connect in Rennes with buses to MSM. The 8:41 AM departure from Le Mans provides a considerably faster travel time than the 7:16 and 9:33 trains.