Mount St Michel or Fountainebleau

Fontainebleau while well worth a visit and historically interesting is just another chateau; MSM is an entirely unique and fabulous one off experience. And this time of year MSM will not be crazy crowded. We visited it in late October one year:
https://janettravels.wordpress.com/2014/01/21/mont-st-michel-is-that-a-real-place/
This is one of those amazing once in a lifetime places IMHO. On the other hand, it is really not a good day trip from Paris as it is so far; it would be best to spend the night and not spend a minimum of 8 hours on the road for the visit.

Ideally you would do Normandy and Brittany on another trip rather than trying to do it as a day trip. Fontainebleau is an hour out of Paris and an easy leisurely day trip.

I agree, stick with Fountainebleau.

To get to Mont St Michel and back in a day may be humanly possible, but it wouldn't leave you time to appreciate the destination. If I went, I'd want 2 full days, including a guided walk on the bay during low tide and overnight on the mainland in a hotel with a view of the Mont so I could see it lit up at night. You may enjoy a vicarious visit by reading this wry essay http://habitsofatravellingarchaeologist.com/the-dos-donts-and-maybes-of-mont-saint-michel/

Fountainebleau seems like a reasonable day trip. I know people do take a tour to MSM as a day trip from Paris but I don't see it as a reasonable distance to get there, back, and see something while you are there. I love MSM but would save it for a time when you can enjoy it.

You might check to see if Chantilly is closer or faster than Fontainebleau. I'm assuming you've been to Paris and to Versailles before. If this is your first time in Paris, you have too much to see to go so far. As Rick writes, "Assume you'll return." MSM as a day trip is a horrible idea, even worse than the D-Day Beaches as a daytrip.

We hired a car from Versailles and spent the whole day there easily and stayed at a tiny little town very nearby in Ponseiilon, my spelling may not be correct as it was 10 years ago, just lovely, so no don't even think of doing it there and back in 1 day, it deserves more than that! Awesome place to visit!

I've driven from Paris to MSM - it's not a day trip. If you're going out there you should also spend some time visiting the D-Day sites so I would recommend 2 nights; therefore, Fountainebleau appears to be what best fits your schedule.

This is crazy. Take a plane to Rennes leaves at 9AM,
And then in Rennes take the bus directly to Mt St Michel
It will get you there in an hour and stay the night and then reverse the way you came take the plane back the next day

The flying idea, while it sounds like it would be a fast option, in reality I think it will be about the same time as driving. Commute from Paris to CDG about 1 hour, arrival prior to boarding at least 1 hour, flight 1 hour, trip from Rennes airport to MSM 1 hour+ (could be longer if the departure time doesn't align with your flight arrival)...over 4 hours total. Good idea but I don't think it helps too much.

The trip from Paris to MSM is most certainly not absurd. I'd imagine a great number of people visit it this way. Heck, each time I visited it I was living in Tours----and that's farther than Paris. School-sponsored trips in France make this voyage without batting an eye. If you're not going to get to France any time soon, I'd say check out MSM. You're going to be exhausted at the end of the day, but the memories you make there will be very different from the ones you'd make at Fountainebleau.

Follow your heart, but it's like comparing apples and oranges. But MSM is truly a treat. Look at Janet's beautiful photos on her blog. If that doesn't inspire you to go, I don't know what will!

I agree that MSM deserves an overnight.. I have had that pleasure years ago..however a few years ago my daughter and I did a quick visit to Paris for another reason.. and my daughters only request was to see MSM.. and we had so few days.. so we did a big bus tour..

It was fine. I saw enough... yes, the island is a heaving mass of humans in the daytime .. it IS much nicer in the evening.. however.. my dd was thrilled she got to visit.. so there you go.

Its a long pricey daytrip.. but if you are never going to get back.. or might be years.. then its not a bad idea.