Paris to Versailles then Mt St Michel

It's a good drive, 3-1/2 - 4 hours, from Versailles, so take that into account. A bit longer from MSM to the airport, so no early flight to Rome. There's a regional airport in Caen, flights are likely not direct and not cheap. You can rent a car in Versailles but be aware you may need to walk or take a cab to the auto rental from the palace. Take all that into account and it is a feasible plan. However, here is what I would not like about it. You arrive at MSM late and tired. That's okay, you've got all the next day, and the next morning and evening is glorious (unless it rains). But by the time you high-tail it out of there you've spent two night there. And your Versailles day is maybe cramped or compromised. The alternative is to see Versailles as a separate day trip from Paris and then drive, or take the train to Rennes from Paris and rent your car there. (Paris-Rennes by train: 2:25; Rennes-MSM by car, 1:15). You arrive mid afternoon and spend the night, giving you the evening and the following morning. The either return to Paris or the airport. Incidentally, there are frequent trains from Rennes connecting in Paris to deGaulle. There are even a few direct trains, but they may not work for your schedule.

10 years ago and an online search yesterday verifies it is still there i walked off of the train from Paris to Versailles and no sooner out the door than there are car rental agencies right there.
I suggest usking Skyscanner to see if there is an airport near MSM area that will take you straight to Rome so you don't have to backtrack to Paris.

Driving from Paris Area to Mont St. Michel is easy, it's sort of like driving from LA to San Jose on the 5 (+/- depend on if you want to stop at other towns like Bayeux/Caen or not). Personally I would not spend 2 nights on the island, I'd get in late around 6pm, stay the night, get the heck out before loads of tourists buses around 9:30am-10:30am the next day. And use that extra night somewhere else like Bayeux, Caen or Honfleur...etc. But everyone travels differently. There are two airports closer to MSM, they are DNR - Pleurtuit airport, or RNS - St Jacques by Rennes. Be prepared to pay over 1000 bucks per person if you want to use those airports. Easyjet and other discount airlines which you can get for under 100 bucks only fly out of CDG area. 5 hours should be more than enough to drive from MSM to CDG, unless you get stuck with the toll machine and our out of date US magnetic credit cards. Which I drove that route last August, one of the unmanned machine exit by MSM gave me some issues and embarrassment. Enjoy~

You'll need to return to Paris to fly to Rome. Currently your budget airline choices are Orly-FCO on easyJet departing at 12:45 or Orly-FCO on Vueling departing at 15:20. Flight times might be different in October.

Yes it looks like returning to Paris to fly to Rome would be our cheapest option. My friends and I definitely need to discuss if we want to drive all the way to MSM. Would an evening in MSM be enough to see what we have to see? And I like the idea of spending the 2nd night in another location. Thanks guys!

As far as 2 nights on Mt. St Michel I don't think I would. It really is so small that one night will give you the experience. I would arrive as early as possible so you can see what its like with tons of people and day trippers, then when they go you have the place to yourself. Unless you just want to relax with friends I would move on... say Bayeux or some other location that you can enjoy for one night. But... for lots of people (me included) it was well worth the drive for the experience. I was there in Nov. so it was definitely unique.

Go to Versailles then on to Honfleur.....great town for girlfriends ! Beautiful walking path along the water, two hours from Paris. We were there in October, and I absolutely loved it....look at pictures of it on-line, as it is beautiful. wonderful seaside restaurants and lots of very nice shops. Great for walking. You can also, in ten minutes, be at the towns of Dauville and Trouville (spelling ?), two other nice beachside communities with lots of shops and restaurants in Deauville. We drove there and walked along the beach. We did Honfleur for two nights and then went to Mt. St. Michel for one - if we had to do it again, would just go to Honfleur. We stayed at a really nice bed and breakfast in Honfleur, with Amanda and Craig as our hosts-found on Tripadvisor, and it couldn't have been nicer. If Honfleur was in the US, my girlfriends and I probably would have been there I'm sure. Mt. St. Michel was very nice, but it's further out. We arrived at 4, and left by 10 AM, and that was enough. Restaurants on the island itself are not very good.