Verdun, in that case. Its a bucketlist top few. Not always easy in the detached almost Disneyesque sort of way that so many sites of tragedy become. The battle area is quite extensive - you keep coming across stuff like gun barrels in farmyards or seemingly miles of little one man pillboxes.
Trier, capital of Roman Germania, well at least one of.. Nice town.
Aachen, where Charlemagne is buried in the Dom. His inability to nominate one of his three sons to be his heir could be seen as the reason for the successive centuries of conflict between France and the fracturing Holy Roman Empire/Germany.
Nurbergring? Fancy trying your luck?
Roads towards it from Germany are excellent. Leaving northwards into Belgium - if you go that way - beware the "road of death" as it is often called by the 'Ring boys. Almost the worst road I have ever ridden on in western Europe.
Down as far as Alsace, the Balon d'Alsace. Old parts of Colmar are nice. Travelling back up the Rhine, there are occasional bits of Maginot Line. No doubt someone will pipe up if they know with more exactitude. You could have a coffee/beer in Neuf Brisach, an pretty much complete Vauban "star" fort and a unesco WHS.
If you have come this far, then Mulhouse is not much further. The French National Motor Museum, which incorporates the Schulmpf collection of Bugattis, confiscated by the French govt after they discovered the Freres Schlumpf had defrauded them of millions in tax revenue in order to fund the world's largest collection of Bugattis. Lots of other stuff with motors too. Did I mention they have Bugattis?
Almost next door is the French Railway Museum. Not so extensive as the UK one in York. It does show how advanced much of their system was, and how homely in parts.
Both have free parking right outside. I got lost last time I went because I was trying to get to the old site. Probably a satisfactory use for a satnav.
Connecting any or all of the above can be easily done by seeking interesting looking roads as they appear on a paper map. The cheap Michelin Regional series, 1:200,000 are more than good enough. The red 1:1000,000 series are really only of use for plotting general lines of advance, IMHO.
Any road which fails in the ride satisfaction stakes usually becomes a road not travelled at all. There are loads to choose from. Finding your own can be very satisfying.