Experienced German traveler here. I was a German major at A&M and lived there for a couple of years.
1. GET OUT of Frankfurt at any chance you have. It's the worst city in the country for history. Unlike most cities that rebuilt to almost a creepy reincarnation of their former self, Frankfurt rebuilt itself as Houston, with glass skyscrapers, etc. Don't spend a minute there if you can go somewhere else.
2. Go to Heidelberg. Not really huge on WWII history, but it's the closest piece of awesomeness to Frankfurt.
3. Consider Nuremberg. It's the other way from Bastogne, etc. but there is a whole bunch of history there. The Nazi Party rallies were there and much of that land is still open, undeveloped and viewable. And the awesome town of Wurtzburg is halfway there.
4. Luzembourg is cool if you can do it. Cologne is awesome if you have time.
But it sounds like you have a short trip. Look at my advice and others and pick what you think works best. Good luck.
