Paris to Mont St-Michel

A day tour from Paris is gruelling (6-10 hours of round-trip transportation time depending on mode of transport) and ensures you see the Mont... at its worst, packed with daytrippers. Much better to spend the night there or nearby (Pontorson). I wouldn't drive from Paris, you could do it by public transportation all the way or (my choice) take train to Rennes and rent a car there.
How late in the fall, btw? Days get quite short by November.

If you look at Google Maps, you will get an idea of what a long trip this is. Naturally, a tour bus would take a little longer than the "Car" icon estimate provided by Google Maps. Using the train, you also have to consider (depending on your itinerary) that the train to Pontorson and the train from CDG airport use different rail stations in Paris. (However, the Paris Metro can get you quite close to all train stations. It's just extra bag lugging if you do it at the beginning and end of a trip. You cannot sleep in Pontorson and fly home the next day - it's too far and too risky.

You should evaluate just how important this excursion is to you, and what are the chances that you will make another trip to France in your lifetimes. Then you can make an educated decision about how to go about the add-on to your cruise. I mean this in a nice way, but if you are like many people who ask this question here, isn't Omaha Beach equally important to you? Should you arrange multiple nights to cover both places? Just as one, rushed, limited-time idea, the Churchill Hotel van-daytrip to MSM from Bayeux is often mentioned here for people who don't have a car, but are going to the D-Day Beaches. (That has its own limitations without a car. You only asked about MSM, I know.)

Unlike most posters on this board, we found 4 hours at mid-day an adequate time on the island itself, at mid-day in the summer. We had a car, and were sleeping that night in Dinard, where we had already deposited our bags (car theft safety issue) in the hotel. Our next car stop was ouside Bayeux, for three days of the Omaha Beach area, and Bayeux itself. I disagree with the advice you are about to receive that you must spend the night on the MSM island or the trip will be a waste. It's a matter of opinion, and of how many medieval towns you have visited.

If the choice is between "a day tour from Paris" and "taking a couple of days and staying near MSM," I'd definitely recommend the latter. Not because you "have to" spend a night on the island, though that would be great, but because the Paris-MSM-Paris travel time is so long. It's been many years since I was there, but another reason (besides midday crowds) to stay awhile would be to see the place with the tide both in and out -- surrounded by water and by mud. Unlike the original visitors, you don't have to time your visit for a low tide, but you might appreciate the experience more if tidal change is part of it.

I like the idea of a fast train to Rennes and renting a car, but I can't speak from experience. Rome2rio is a good way to compare transportation options, though it's better to do actual bookings with the actual sellers.

Everyone is pretty much going to say do an overnight trip to Mont Saint Michel but if you do not have the time and want to do a day trip, do a bus tour from Paris. That way all the driving is done by someone else. Last year, friends who were staying in Paris had Mont Saint Michel on the husband's bucket list and did it as a day trip and found it very enjoyable. He got to see a place that he fell in love with as a boy when he saw a painting at an older relative's house and that was worth it for them. They used Paris City Vision.

We went to M. St-Michel from Bayeux. After visiting the Mont, limboing all the way up, etc., we preferred the view of it from the mainland.

If you can allow for 2 days or more, it is more pleasant to spend a night in Mont St Michel or in a place nearby (St Malo, Rennes, Pontorson, Fougères...) and then enjoy MSM at a time when it is not crowded (even if it mostly matters from May till September).
If you want to do a day trip from Paris and wish a more personalized trip than a big bus tour, then consider a TGV train to Rennes (1.5 hour) or St Malo (2 hours) and do a private tour from there to MSM with a driver (Uber, VTC).

Wouldn't advise a day trip - too far. We spent 2 nights there in 2014 and enjoyed visited MSM as well as Cancale. We drove from Paris and also visited the Normandy beaches, cemetery, etc. on our way.