You waste so much of a trip traveling from one place to another and anywhere I've been in Europe has some amazing sights that are off the beaten track for Americans, but there's something awesome about being in a castle practically by yourself instead of fighting the mobs at the usual tourist sites.

If the kids like history, consider Provence. There's tons of Roman and medieval stuff and really just an amazing number of things to see.

I know France a lot better than I know Italy, but I don't think you can go wrong with either country.