France

Reims would be another choice, several champagne houses where you can learn how it's made.

Strasbourg

Reims is worth a stop. There's a great Cathedral, some Roman ruins and champagne houses (I chose Mumm's because it makes the best wine). Another town in Champagne would be Troyes but that's not nearly as convenient.

Edit: also a nice pedestrian cafe area. That's where I stayed.

That's kind of hard to answer without knowing what part of Germany you'll be coming from. I imagine the answers would be different if you were coming from Hamburg or if you were coming from Stuttgart. Also, I would recommend Strasbourg if you were coming from Munich, but I would recommend something further west, closer to the midpoint if you were coming from Stuttgart. Also, mode of transportation could factor in to recommendations.

Yes, are you travelling by car or train?

We will be traveling by car. We will travel along the Rhine and most likely will not go any further south than Heidelberg before going back west toward Paris. A friend recommended Metz. Is it worth a look? Most likely we will only spend an afternoon and one night someplace before heading on to Paris.

Metz is quite nice, with a small interesting old tower (view only) and a nice park, and nice waterfront, and a nice park in the centre with an interesting very long fountain.

If you are that close to Luxembourg go there, either city or country. For a lovely park and good atmosphere, consider Monfort les Bains near Schengen.

Oh, December - maybe Luxembourg City then, outdoors in the countryside while gorgeous in Summer may be a bit drab, wet and mucky in December.

The Battle of the Bulge museum in Diekirch is excellent, telling the story from all sides - the Germans, the locals, and the Allies.

From whence in Germany, please?

Do you have the car nailed down? If a car from Paris it will likely not come with the snow tyres required in Germany. If a German car it will have a hefty drop charge to repatriate it.

Yes, we have already reserved the car. We will be traveling through Belgium and the Netherlands before Germany. Can we request a car with snow tires? From what I've read I thought it not all that likely it would be snowing around the Rhine the first part of December. But....I'm just going off of what I've read having never been to Germany.

Metz is closer to the halfway mark.
You’ll run into problems with the car on two counts. First, a hefty fee is levied if a car is dropped in a different country than where it was rented. The Rhine to Paris or even Metz and Paris is an easy train trip, if that helps you avoid the charge.

Second, those snow tires are required by law in Germany. Are they required where you are picking the car up?

Thank you for the info. We are thinking now we will cancel our car in Paris. We will train to Belgium and Amsterdam. Train to Germany and rent a car there and just turn it in before we leave Germany. We would just train to Paris. Any ideas on where to rent and drop off a car in Germany? We would like to stay somewhere between Cologne and Koblenz or Mainz most likely. We won't go any further south than Heidelberg.