Highway travl from Charles deGaulle AP to Mont St. Michel

What were your touring plans for the 3 days you'll have at Mont Saint-Michel? If you are planning to branch out to Cancale and/or Saint-Malo in one direction or toward the DDay sites you'll need a car.

I was planning a day trip to the D-Day area but was thinking that could be hired from Mont St Michel -- perhaps a shared mini-van type of tour.

From Roissy CDG airport to Mont Saint Michel, it would be cheaper to consider a direct TGV train from CDG (the train station is right next to the airport) to Rennes and from Rennes either take a bus or a taxi/Uber to MSM. Once in MSM you could take private tours both to Brittany or Normandy.
But spending 4 nights in MSM seem quite much : for 1 night it may be pleasant, but be aware that there is no particular activity there at night. It might be more pleasant to divide your nights in 2 places (St Malo, Cancale, Granville, Bayeux...).

To my opinion your 4 nights stay will be more interesting if you can explore the area around Le Mont-Saint-Michel and for that you will need a car. Arriving with the train you can rent one in Rennes for a few days or more and drop off for instance on the way back for catching the train to Paris in Bayeux. It’s a much used base for visiting the D-Day beaches, if that is the plan is ofcourse, just a matter how you like to organize it.

Just a few ideas, like the drive along the coast between Cancale and Saint-Malo is very scenic and worth considering is Fougères, some 50km south-east of Le Mont-Saint-Michel.

I agree with the previous poster. Take a TGV train direct from CDG to Rennes and rent a car at the Rennes train station. Driving to Mont St Michel takes about 1.5 hours, and it's pretty easy to get out of Rennes. Four nights is probably too long, but as others have said, you can visit St. Malo or even, for a long day, Utah Beach. If you want to visit the D-Day beaches on your way back to Paris, keep the car since it's the best way to get around. If you don't want to drive all the way back to Paris, drop you car off in Caen, where there are four rental agencies across the street from the train station.

Adding onto the original question, if I rent a car in Rennes (which seems like the best option for our September trip,) where can I park it when I spend the night on Mont Saint Michel? We are staying at a home through Airbnb.

Many thanks for all your excellent advice and comments. After considering it all, we've decided to rent the car at CDG after all and drive to Mont St Michel. We will stay 2 nights at the Le Relais du Roy then drive up to Bayeux for 2 nights then back to Paris for a week. While at Bayeux we will look for a D-Day tour and plan to visit the museum and American Cemetery. While in Paris we are planning to visit the usual attractions - Eiffel Tower, Arc de Triomphe, Louvre (2 days), Versailles and more.

Is there a need or benefit from having a laptop computer while in France? I have an Android smart phone which seems to be capable enough for most needs.

I always take my laptop and use it to store photos. While many websites can be accessed on the phone, the laptop is much more convenient.