Planes, trains and automobiles

It depends on if you want to get there as fast as possible or make stops along the way, and where you want to make those stops.

But flying from Avignon to Paris is a really bad idea, it is 2:40 by train. Flying will take longer and cost more.

With just 3+ days I don’t think you have time to drive and stop to see anything in the Loire Valley. If going from the southern part of France to the extreme north to see Mont St Michel, your one checklist item before you fly home from Paris, I think I’d take the train from Avignon to Montpellier, fly to Caen, rent a car at the airport, drive to Mont St Michel, spend the night, drive to Bayeux for the night then return the rental at CDG for your flight home. If your flight is at all early and with a 3-4h checkin, you may need to spend your final night at one of the convenient airport hotels after returning your car the night before your flight.

EDIT: It’s a big effort to see one thing on the opposite side of the country on this trip. Perhaps you’ll return to France for some time in Paris and Normandy in the future?

I imagine a 15-day river itinerary would include the Seine and the Rhone, and a train linking the two, right? It would be much easier to hit Mont Sant-Michel while you're already in the north. Maybe you could move those three days to the start of the trip?

To put this in perspective, that is like someone coming to visit New York City and saying I just want to see Indianapolis before heading home from Saint Louis. It is about the same distance, and at least as difficult to get from Avignon to Mont-Saint-Michel, and then Indianapolis to St. Louis.

Thank you for the help! Back to planning, the fun part!

Although I consider that this might not be reasonable from a timing point of view, but since you enjoy the fun of planning, note that from Montpellier airport (1 hour drive from Avignon) there are flights to Rennes with Transavia (twice a week).
Once in Rennes you are 1 hour drive from Mont Saint Michel.

From Montpellier there are also flights to Caen with Volotea (Also about 1 hour drive from MSM) twice a week.

I might do it this way… TGV from Avignon to Paris, TGV from Paris to Rennes. Spend the night in Rennes, next day drive rental car to Mont St Michel. Spend second night in Rennes, then TGV back to Paris. I love taking trains in France and i hate flying so trains are my preference… relax and watch the beautiful scenery out the window.

I agree that the TGV is a better option from Avignon to Rennes. I see no advantage in flying, and the train will be more comfortable and if booked in advance cheaper. You have to change station i Paris though, but that is not that bad.