Normandy/Brittany loop

You had me until that last sentence there. 11 year-old complicates things.

You don't mention if you've been to Paris or France or Europe before.
Has that 11 year old? What are his/her interests (besides not being in a car...)?
Also, what time of year?

Ah, yes. A little background. We have been to Paris several times but not the 11 year old. He has been to Tuscany and Venice and did quite well, although we stayed in the same villa for two weeks (minus the over night to Venice). I'm trying not to let his disdain for the car completely rule the trip though, so take that into account. This morning I realized maybe I should return the car in Rennes and take the train back to Paris to enjoy some more time there at the end of the trip. Have you been to Carnac? I am really curious about it and I think it might impress an 11 year old.

Time of year is March. His interests include WWII. This trip was his pick. D-Day beaches being the focus.

I’d recommend visiting Arromanches to see the remnants of the Mulberry Harbor. For an 11 year old interested in WWII, I think the museum there would be more interesting than the one in Caen. I would also recommend taking a tour of DDay sites rather than just traveling point to point. If it’s not on the tour, make sure to visit Point du Hoc. An overnight in the walled Breton town of Dinan might also be nice.

March is not ideal for a windy, chilly coast. The child might enjoy St. Malo as much as Dinan. There are quite a lot of Megalithic rock sites to choose from. Probably no Paimpol steam trains in winter. Not much point in stopping at Giverny on the way back, but maybe Chateau Chantilly. Interested in Rouen or Chartres?

Chartres is certainly worth a stop, even a picky 11 year old has got to be impressed by the cathedral.

Suggesting overnighting on Mont St Michel itself, rather than day-tripping to the island; it's worth the expense and hassle to be there late evening and early morning. The kid should love it (everyone get up super early).

Rouen is our first stop outside Paris, where we will pick up the car. Unfortunately, Giverny will not be open to the public yet for the year. VERY disappointed about this.
Hmmm, maybe we should consider adding Chartres.
Thanks for all your suggestions. Keep 'em coming!

Visiting Normandy and Brittany is actually a good combination but if you want to enter Brittany further than places close to Mont St Michel (Rennes, St Malo, Cancale, Fougères...), you should allow at least for 4 days (you could spend weeks following the coastline or discovering towns inland!).
Carnac and the surrounding area is actually a unique place for megaliths (high concentration of different kinds). It is located on the south coast of Brittany, so it would be worth spending at least 2 days in this area (Vannes, gulf of Morbihan...). Otherwise there are less touristic megaliths and more isolated ones in the countryside (Roche aux fées for example).

An 11-year-old would find Carnac fascinating, so I would seriously consider it. Also, Fort la Latte near St-Malo is another kid-friendly sight with actors recreating medieval crafts and warfare. To shorten your car journey back to Paris, drop off the car at the train station in Rennes and take a TVG.