Has anyone tried taking a day trip to Mont Saint-Michel from Paris? Taking a RS Tour which starts in Paris and wondered how doable a day trip like this would be. My first time in Paris so am trying to prioritize.
Its theoretically possible, taking a combination of trains and buses. But travel time at both ends is long, leaving not a lot of time actually at MSM. And you would arrive amidst the heaviest crowds of the day, slowing you down even more. I wouldn't recommend it.
Is this additional time before/after the RS tour? Or a free day within the tour. It's really too much for one day, but we took the train from Paris (Montparnasse station) to Rennes; picked up a rental car at the Rennes train station (quick and painless) and drove to MSM. (It's a very easy drive; but we stayed the night in Pontorson or you could stay on MSM) for one night.
There are several tour operators which run groups out and back from Paris. After reviewing a couple options, we settled on City Wonders' itinerary earlier this year:
https://citywonders.com/paris-tours/mont-saint-michel-day-trip-from-paris-tour
It is a FULL day; approximately 4 hours' travel in each direction (stopping for a break midway to stretch legs and get a coffee), with ample time once there to enjoy the Abbey and have lunch in the surrounding village. So in answer to your question, it is very doable, but plan on being at the departure point around 06:30 and getting back into Paris after 18:00.
Enjoy your RS Tour!