Mont Saint-Michel is tidal island and a UNESCO World Heritage Site located off the coast of Normandy. It’s about a 4 hour drive or train ride from Paris. The island is rich in history, dating back to the medieval times. It’s probably most famous today for the abbey that sits on top with its stunning, gothic architecture.
Although it seems big from the outside, a couple of hours is enough for you to explore. The only negative fact for me is mass tourism, a lot of people come there and the streets are very crowded.