We'll fly into Frankfort in May and thought of heading out to Paris by train very soon thereafter but we'd like to stop somewhere along the way for a night. I heard that Leuven, Belgium is nice. Any ideas? WW2 Sites are possibilities too.
I would track down a rail map or play around on Bahn.de and start there. Ideally, someplace along a main line would be best if you are looking at one night. Leuven would not be too bad, looks like about 4 hours to get there from Frankfurt, then another two hours (thanks to fast trains) to Paris. Basically this takes the route of north to Brussels, then to Paris. Another route would be more direct into Paris, options for a stop would be Strasbourg or Reims and the Champagne Country. There is obviously the Rhine, but that is pretty much just outside of Frankfurt. Koln is a little further, then Brussels of course.
You could stop over in New York. It's between Frankfort (the capital of Kentucky) and Paris. Then you could fly from there to Paris (there are no trains from from Frankfort to Paris).