Normandy

Do this right: go in June, be there for D-Day. It's OK: the locals haven't forgotten. You're welcome there any time, but being at the beaches on June 6, visiting the cemetary -- well, ideally every American should do that.

Or, treat the beaches as beaches, if the weather is warm enough. Have a few days of relaxing in a proper seaside town. Or, a harbor town, and go nuts on fresh seafood. Sole meunière alone makes the trip worth it (with a glass of local cidre).

Mont St. Michel is a part of Normandy, and you have to see it at least once. Generally, Normandy is a bargain, but not here: for the full experience, be prepared to spend -- and spend the night on the island. 

And certainly you have to see Bayeaux, and The Tapestry. Read up on it, prep your kids, and go over the whole darn thing. It's such a great story: there's a reason why everyone knows the year 1066, the repercussions of the Norman conquest have lasted 1000 years.

You'll likely fly into Paris to get to Normandy; Brittany and even the Loire Valley are close by.