I agree with this wholeheartedly. I think the fact that Heidelberg once was the site of a large US military base has a lot to do with its status on the US tour itinerary. Many (certainly not all) servicemen stationed overseas prefer not to go too far afield in exploring their host nations, and I suspect that's why many of them came home with stories about how you "must" see Heidelberg. Not that it's dreary or in any way a bad place to visit; it's just not at the same level, as, say the Neuschwanstein castle or Rothenburg o.d. Tauber, for instance, the way it's often portrayed.

Speaking of Rothenburg, isn't that within the 2-3 hours circumference of Frankfurt that the OP imposes? Maybe not, but worth considering if that 3 hours radius could be stretched at all. And what about a trip down the Rhine (between Bingen and Koblenz) and up the Mosel, ending in Trier? Or the reverse of that itinerary? Maybe start in Trier and spend as much time as you wish there, then head down the Mosel and when you reach Koblenz, if your time is not up, then you can head up the Rhine toward Bingen. That's exactly what I'd recommend for a first time visitor to that area with couple of days to spend.