Johnny W.
Yelp
Literally, this is a city within a castle! Since the 8th century AD, this has been the seat of the monastery from which it draws its name. One of France's most recognizable landmarks, visited by more than 3 million people each year, Mont Saint-Michel and its bay are on the UNESCO list of World Heritage Sites.
If you're staying in Paris, you can take a train to Rennes, where you can take a direct shuttle bus to Mont Saint-Michel. When my friends & I went, we rented a car from Paris, and drove to Normandy. It was a great decision, as it was very convenient for a group of us to share the cost of renting one vehicle, and we had the freedom to drive to Bayeux, visit Omaha Beach & the Normandy American Cemetery, and drove to Mont Saint-Michel, and back to Paris. If you decide to drive, they do have a large parking lot, and a shuttle that will take you to the bridge in front of Mont Saint-Michel, where you'll just need to walk a little from there.
Be sure to wear comfortable shoes though, as you will do a lot of walking around this castle and it's town, with a lot of steps too! You can easily spend a whole day here including the commute, so be sure to give yourself extra time to enjoy all of the interior & exterior of this gorgeous historical landmark. The views are amazing and a must visit when you're in northern France!