We are actually heading back to Paris this Friday!!! Mont St Michel is a doable day trip - but can be tiring. We actually did it as a day trip from Paris 2 years ago on July 31, 2019 - I had to pull up my notes. We took the SNCF from Paris to St Malo at 8 am - got into St Malo at 11 am. Met our tour guide at the train station in St Malo - who showed us the the old walled city of St. Malo first, then on to St Michel and then to a small village Dinan which has a theater dating back to 1224. Our tour guide then returned us to Rennes to catch our train back to Paris. The tour was myself, my husband, and my sister. We booked it through LinkParis.com. Our tour guide was Marc Le Meur through Westcapades and he was fantastic! We had time to wonder through all 3 sites/cities. He gave background on the background of Mont St Michel and the disagreement on who claims its - Normandy or Brittany. We took the backroads. In 2019, the personal tour was just under $500 for all 3 of us and the train wound up running 270 Euros for the 3 of us. In 2019, it was crowded at St Michel and you felt like you were part of a moving river going through the monastry. But in this time of covid and the number of tourists down almost 40% - I am not sure of the volume of people now who might be going through Mont St Michel at one time. I would have enjoyed a bit more time on the coast - but not having our own transportation, I would not be comfortable trying to get around to the different sites we saw. If Marc is still giving tours - I would highly recommend him. Again, this is not a day trip for small children - but teenagers should definitely be able to handle it. If you go, the buttery cookies are wonderful in that part of France. The food - well, knowing we would be a bit rushed, we were not expecting to have a 2 hour lunch or dinner any place - so we did not mind sandwiches with real butter, cheese, and ham from a local shop. I don't believe we would have gone on our own to Dinan - so I am grateful we did with this tour. Mont St Michel has always been on my bucket list - so even if it happened with a very long day trip - it was well worth it to me. My husband, sister, and I are notorious for the long day trips - we try to visit France at least twice a year and always make a point of planning 1 or 2 day trips to either a place in France or other European country. We don't mind getting up early or coming back to Paris very late in the evening.