Mont Saint-Michel Mont Saint-Michel is special for so many reasons. It’s sort of an island, or at least used to be. It was a tidal island, meaning that it’s an island during high tide and connected to the French mainland during low tide. There is currently a land bridge, but the French government has initiated a huge project that will wash out the silt that makes the land bridge to restore Mont Saint-Michel as an island. It is also a pilgrimage destination. Archangel Michael appeared to St. Aubert in the 8th century and ordered him to build a church on the island. As such, the Mont used to be a major pilgrimage destination. Now it’s more of a tourist destination, but there are still around 100,000 pilgrims each year who cross the bay on foot. (This is allowed only with an approved guide). 
Mont Saint-Michel is an architectural wonder. As you're arriving on the bus, you see fields and nothingness and then, boom, there's an all-of-a-sudden amazing town rising offshore in the distance, atop its own island. It looks like an enormous, thousand-year-old birthday cake rising from the sea. Layer upon layer of buildings constructed in different styles at different times.
When you arrive, don’t be too quickly turned off by what you encounter at the bottom—tacky tourist shops full of mass-produced plastic toys and other “souvenirs”. You will also find several expensive, tourist-gouging restaurants that don't seem nearly as appealing as their prices might imply.
But then you get past all that and climb and climb and climb. There’s no lazy-bum version of this (trolley, funicular, etc.), so bring your walking shoes. At the top, you’ll visit the Abbey that you see from the distance as you approach. The entire walk up is one vast lookout over the Channel, but at the same time the fun is in all of the climbing. You can slowly explore narrow walkways, stone stairs and hidden side alleys. 
How to get there? Take the train from Paris Montparnasse to Rennes and then a bus to Mont Saint-Michel. The bus company is Les Courriers Bretons. You can even do it as a day trip by getting the earliest train there and the latest train back. Every day other than Sunday, that would mean leaving Paris at 7:05 am, which gets you to Mont Saint-Michel at 11:00 am and then leaving at 4:45 pm, getting back to Paris at 8:40 pm. You can buy the train and bus tickets at the same time at Montparnasse. Show them the "Courriers Bretons" bus schedule and explain that you want to get all of the tickets together. Then there's nothing to worry about in Rennes—the bus station is just outside of the train station and over to the right. 
For lunch, I suggest you bring your own picnic from home (with lots of water) to avoid the high-priced tourist trap restos. (Thanks to Alan Coyne for the photos!)
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