Normandy in 2+ days

No comment on MSM, except that we enjoyed our day there. As for food & drink, our best meal in Bayeux was at L'Assiette Normande, based on a tip from our apartment rental landlady. It's across from the Cathedral; tiny place, a reservation might be a good idea if you are interested. http://www.lassiettenormande.com/

If you do drop MSM, we can recommend a leisurely ride thru the Cider/Cheese routes. http://www.calvados-tourisme.co.uk/en/discover/tourist-trails/the-cider-route.php It might even work as a detour en route back to Paris. Bon voyage, September should be terrific in both locales.

Rather than staying at a hotel "near" CDG, stay AT CDG there are several hotels right at the terminals fitting any budget: IBIS, Novotel, Hilton, and Sheraton, to name four. Just be sure you look for the ones that say "CDG Terminal" as there are several Ibis and Novotel properties in the vicinity. The CDGVAL train connects the terminals and will get you to your check-in site quickly. I can personally recommend the Novotel.

I've been to MSM once, and D-day area a half dozen times. If "his focus is really on DDay sights," and MSM is "not your thing," I'd drop it. You'd spend your 2nd Normandy day doing a lot of driving, and being with the crush of tourists. And there is tons to see in upper Normandy.
That being said, I'm going back to MSM in September. But I'll be in Normandy Thursday through Monday, so I have more time than you. And I'll spend a night on the Mont when I first get to Normandy (I hear it's a much richer experience). So I'll still have 3 more days and nights to see other things in the area.
If you squeeze MSM in, I would do what I am doing, which is go there on your way out and arrive later in the afternoon when the crowds subside a little. You can then drive to Bayeux in the evening (1-1/2 hours?).
I've both driven to Normandy from Paris, and picked up a car at either Caen or Bayeux. If you're not jet-lagged, driving will probably get you there quicker. But there is something peaceful about train travel IMHO, so I'm taking a train this time to Rennes on my way out, and back from Bayeux. I've always dropped off a car in Normandy and taken the train back. Caen is super convenient with rental car places right outside of the station, plus there are more direct trains in Caen. I've dropped off a Hertz car in Bayeux, and the drop-off is actually a service station on the periphery road. But the attendants will give you a ride to the station. I once dropped of in Cherbourg. It's a little bit of a drive. But it was a pleasant walk and coffee along the inner port to a station at the end of the line (meaning you're not rushed to get on the train). And it seems like a more real town than many of the more touristy areas.
Good luck whatever you decide!

If you're likely to visit a different and exceptional medieval site on a future trip, I don't think you have to drive all the way to MSM. Depending on his interests, some other options include St. Malo, Vitre, Honfleur, Giverny, Chateau Chantilly, Rouen, Coutances, Fougeres (sorry for all the omitted accent marks). I'm not saying those are equivalent to MSM, I'm saying they are worthy and special places that might be of interest to the two of you, with a car. One reason for Vitre is that it has some of the visuals of Dinan without the long drive. It also has a nice castle (exterior only, I think.)

I don't go for the Sergeant Rock tours, but I wonder if you should ask Overlord for some of the major sites you are sure to visit, so you can make a list and choose places to take the car to cover the rest of the war. There is so much to see that you should plan this in advance. Use the search box above for Caen and car rental ideas in past posts. (We drove from farther away ... ) Note that there are a few beachfront restaurants that might save you some time backtracking (and provide a toilet while far from the car.)

Edit: Here's just one past thread I ran across:
https://community.ricksteves.com/travel-forum/france/normandy-itinerary-advice-d52d2478-0183-487f-a9f3-6a8d8fb23131

This next thread includes my list of actual "substitutes" for MSM ... ....:
https://community.ricksteves.com/travel-forum/france/mont-st-michel-or-versailles

you have options that can satisfy both of you. My general plan would be to take a morning train from Paris to a city such as Rennes or Rouen, explore the city and have lunch, pick up your car and do sightseeing on way to Bayeux. Next day is D-day tour. On Fri more sightseeing then drop the car off outside Paris and return by train.

So if you went to Rennes (a great city!) after lunch you could visit St Malo, have a few oysters on the dock in Cancale, then show up around 6:00pm at MSM. A big plus of this (not known by many) is that if you eat at one of the restaurants across the bridge you can park your car right in front and entrance is free. I highly recommend Le Relais du Roy, the meal I had at this restaurant is one of the best I've had in France! Look up their reviews. Restaurants close early on/near MSM so if you're doing this get there no later than 7pm, and check on the free entry hours. Then after dinner take the free shuttle to MSM and spend an 1-2 hours exploring, then go to Bayeux to sleep.

Going the other direction you could go to Rouen, then to the cliffs at Etretat and dinner in Honfleur, then on to Bayeux.

RCA - would the Cider/Cheese tour and Giverny be possible in the same day. Then drop off the car and head into Paris? Is that feasible or too much on the plate?

Thank you all for the great ideas! I need to map them out now. Does anyone know if it's an enough drive from MSM to Bayeux to do at night?

Larlock, I just replied to your question in a PM.

Caroline, since you're renting a car, you must use Google Maps or the Michelin site to get a ballpark-estimate of each driving jump you are planning. Then add 20% for unfamiliarity, construction, and confusion. Some people care about tolls, especially because reports of credit cards being rejected at toll machines. I think we just paid cash.

Here is a link to the restaurant I mentioned: https://restaurants.le-mont-saint-michel.com/en/ It looks like parking may have changed slightly, when I was there 2 years ago the gate was simply open at nite, now it looks like they charge 4E. But they specifically mention that if you make a reservation at their restaurant you can park, dine, and use the shuttle to MSM. So perhaps you could get there around 4-5pm, visit MSM, have dinner, and be driving to Bayeux by 8pm. It would be a bit of a rushed afternoon but it fits in the visit to MSM.

Edit: I have rethought the advice I gave above & earlier. Sometimes less is more. If I was doing this I'd leave Paris on the 8:15am train and be in Rennes at 10am. Drop your bags at a participating hotel (see http://blog.nannybag.com/en/luggage-storage-rennes/) or at the TI (http://www.tourisme-rennes.com/en/practical/luggage-rennes) and walk around town, have lunch, visit a museum, walk thru beautiful Parc du Thabor, shop. Head to MSM in the afternoon and arrive by 4pm, park in front of the restaurant. Google says its a 1:15 drive. That gives 3 hours to visit MSM which is plenty. Sit for dinner at 7pm. At most 1 glass of wine for the driver! Leave around 8:30pm and its a 1.5 hour drive to Bayeux. Count on 2 hours including finding the hotel. If you bring your own GPS the trip isn't hard; I would not want to be reading roadsigns and fumbling with a map in the dark.