Rouen or Mont-St Michel?

D-Day Beaches in December ?
Because I'm not one for military history, I would choose Mont St Michel and Rouen, skipping the beaches.

I think the distance is too great between Rouen and Mont St Michel. We are American and do want to see the D-day landings, at least a few and see the cemetery.

Would this work? Perhaps not with the short days of December.
Day 1 - Long travel day from Munich to Rouen (1 night)
Day 2 am - In Rouen
Day 2 pm - Pick up hire car for D-day beaches and cemetries. Finish in Bayeux (1 night)
Day 3 am - D-day beaches and cemetries
Day 3 pm - Drive to Mont St Michel (1 night)
Day 4 am - Visit Mont St Michel
Day 4 pm - Return hire car in Rennes and train to Paris.

Having to choose, I would pick Mont St Michel. Rouen would be better during summer when you could include Giverny as well.

We did a day trip to Rouen from Paris and it is an interesting place quite different in architecture from Paris but I would chose Mont St. Michel in a heartbeat, especially with kids. This is a place they will always remember (assuming they are school age and not toddlers) Mont St. Michel is stunning at a distance and I am betting it will be quite deserted in December; we went in late October and there were few tourists. We stopped for hot chocolate at a place near the entry gates before making the climb to the Abbey.
https://janettravels.wordpress.com/2014/01/21/mont-st-michel-is-that-a-real-place/

We also did the beaches on that trip. I would definitely visit Pointe du Hoc and perhaps Omaha Beach and the Cemetery. And be sure to see the Bayeux Tapestry.

It sounds like you have seen some train schedules, but just a reminder that you will change stations in Paris, as well, something like this:

München Hbf Fr, 04.12.15 dep 09:46 EC 390
Stuttgart Hbf Fr, 04.12.15 arr 12:00

Stuttgart Hbf Fr, 04.12.15 dep 12:55 11 TGV 9574
Paris Est Fr, 04.12.15 arr 16:37

Continue by Taxi or Metro

Paris St Lazare Fr, 04.12.15 dep 17:30 IC 13115
Rouen-Rive-Droite Fr, 04.12.15 arr 18:50

Note: If you are using a rail pass and if the pass does not include Belgium, then don't choose a route using the Cologne-Paris Thalys train through Belgium. If your pass does include Belgium, the seat reservation fee is $25-35 per person.

I would also consider flying from Munich to Paris (such as TransAvia to Orly or Air France to Charles de Gaulle) and picking up the rental car at the airport.

Thank you all for the great responses! We are considering several options based on your responses... Checked airfare and trains to or from Rennes. Loved the blog on MSM. We want to see as much as possible without making ourselves or kids crazier than we already are. I originally thought we would find a place to stay for three nights near Bayeux. Now we are looking at options that decrease back-tracking. I like getting somewhere (especially with kids) and having it as a base. But with limited time in the area, it might make sense to spread out our stays a bit...Arrive at one train station and leave from another to return to Paris. We shall see...