Feedback on Possible Itinerary!

We found Mont St. Michel to be a disappointment. The best part was the view as we were walking up to it. So, with that said, I would suggest focusing on the Bayeaux area and the D-Day history. That could easily take up your time. Will you have a rental car? I highly recommend that if you are exploring the beaches and museums. We have been to that area twice, and have never run out of things to do. On both trips, we took the train from Paris to Caen and rented a car there. Several car rental agencies are next to the train station. Pay close attention to the days/times that rental agencies are open. On our first trip, we hired a private guide for a half day. He gave us a terrific overview of the area, which enabled us to then explore on our own for the remainder of the trip. For us, that was well worth it. Having not been to Rouen, I can't speak to that. Enjoy your trip!

marino.elizabeth,
One never has too much time in Paris! Three days in the city is very little. I also definitely recommend Rouen, and suggest taking an early train there and renting your car next morning on the way out of Rouen. Take D982 along the Seine and stop at Jumiege Abbey on the way to Bayeux. This is a beautiful drive and would take about 4 hours with an Abbey stop and a stop for lunch. You can return the car after Bayeux at Caen (1/2 hour from Bayeux) with probably only a minimal fee for different return location before going back to Paris by train from Caen.
IMO, with the time you have, I might skip Mont St Michel. I also am most impressed with it when viewing it from outside. In Bayeux, drive yourself or take a full day tour of the DDay beaches and museums that appeal to you. Try to be at the American cemetery for the flag-lowering ceremony at day's end. It is very moving. A full day was enough for me, but many people spend days touring the various landing sites, cemeteries, etc. I would take one day to visit cideries (see the Normandy Cider Route) and/or other towns nearby (Beuvron-en-Auge, Honfleur, Ste.-Mere-Eglise, Arromanches, for example). Alternatively, you could take your last day from Bayeux, drive to Mont-St-Michel (under 2 hours) to see it, drop the car at Rennes and take a late train back to Paris. Or drive late in the day to the Mont, spend the night on or nearby, visit the Mont and Abbey in the morning, drop the car at Rennes and return to Paris on an afternoon train. There are lots of configurations in this part of your trip.
Have fun on your trip. And don't shortchange Paris!

"We'd do Saint Michel and/or D-Day Beaches (necessary to do both?),"

It really is a personal choice. I agree with TravelBug and Judy that for me the best view was from the shore walking toward it. Once inside it's a madhouse on the small narrow streets and yes, even mid-October it will still be busy. I've been 3 times on various tours in both spring and fall and it's always been mobbed except for the night I stayed over on a Rick Steves tour when it emptied out in the evening but then there was nothing open and for me it was boring, lol. For others this site is a highlight.

Since you are considering doing WWI sites someone must have an interest in our fights in Europe. The DDay landings area is fantastic in my view. Do book a guided tour for this. It's important to get the lay of the land, understand the defenses the Germans put into place over 5 years and then to see how the Allies made their assault along the coast.

Your vacation, though so it would be your choice.

https://community.ricksteves.com/travel-forum/trip-reports/tr-france-october-2025-bayeux-rouen-paris

This is what I saw last October staying in Bayeux , Rouen and finishing in Paris. There's plenty to do in those 3 cities. We did visit MSM and had 2 full days of d day touring but my grandpa was a paratrooper in WW2 so it was personally more important to me than an average human.

Sorry, I don't understand - if you are into WWI then the areas around Villers-Brettoneux, Lille and Arras, into Belgium, are where WWI battles were fought. If into WWII however, then the D-Day beaches in Normandy become relevant. Big difference, completely different war fought for completely different reasons, at completely different locations.

Lavandula

Elizabeth,
I am a history major and find an interest in both world wars. North of Paris is, of course where you can easily visit any number of sites re WWI. I have been to Verdun 3 times now and always find it moving. Your friend would get a lot out of it. Having a tour guide driving you will allow you to see a few of the many, many battlefields, cemeteries, monuments and museums, without worrying about finding your way. You will have to decide which sites are most important to you, as well as structure a logical route. Trip Advisor may have some tour company suggestions for a day trip. Just remember that there are thousands of sites just in that area. There are many other areas WWI as well. If combined with a champagne tasting it will be a long day trip, but a satisfying one. We have always driven in France so I have no tour guide recommendations.
Your plan for Normandy and WWII sites will give you a nice comparison of the two wars. The sights you will see regarding both wars will move you...I don't know anyone who hasn't been affected by their visit to either war's "remnants".

For WWI, visit Verdun, the huge ossuary is there and the fields still show where trenches were and the area was devastated.
When trees are cut that were living during that war, the sawdust likely contains mustard gas.