OK, here's a couple more suggestions. If you stay around Metz, seriously consider lodging at the Citadel. I's a very good hotel, a converted military building, and the location couldn't be better. Add to that they have parking, which is a big deal in Metz.
https://all.accor.com/hotel/9199/index.en.shtml?utm_campaign=seo+maps&utm_medium=seo+maps&utm_source=google+Maps
Do visit Mont Sainte-Odile.
On the drive north to Frankfurt there are a couple of very nice places you could visit. I highly recommend you come up A35 (France) to where it runs into 65 (Germany) and take that up through the wine region. You'll pass over a dozen castles on the left side as you come up the valley.
Here are some places you could detour to, none too far off the main route:
Annweiler - This medieval town is small, cute, and has a terrific view of Castle Trifels, which is where King Richard was held for ransom on his way to England after the Crusades. You can climb to the castle, but you may not have the time.
Deidesheim - This is like the German version of what Calistoga (Napa Valley) used to be. A locals hang out with Michelin restaurants, high quality lodging, and wine bars all around a 1200 year old town center. The moat is now a garden. A lot of the city wall is now part of the houses. One of my favorite towns nobody knows.
Freinsheim - Just celebrated it's 1250th birthday, and the walls are still standing. You can walk the whole perimeter in an hour or so. Good food, wine, and very few tourists. Perfect for a couple hours stop in a peaceful place.
Grosskarlbach - Have a meal, or just ice cream, at DiBello's. Or go into the pottery shop across from the church, walk all the way to the back, and you'll find you're in a true wine garden. A very small, very secret, jewel of a town.
None of these are going to show in tour guides. Certainly not ones for Americans. But if you Google them you'll see some pictures. And you can tell your family you found places that really were off the main road.