Thanks, everyone!! I appreciate all the contributions and all your expertise and taking the time to write up your suggestions, PalenQ, Bilboburgler, Fussgaenger, Thibaut, and Lubitsch. Lots of great food for thought, and of course not necessarily a consensus, but I can sort that out. I also need to research a bit more the implications of 2 vs 3 bases. I think I still want 3 unless we cut out Germany. While we've visited Germany before (Heidelberg, Black Forest, Rothenburg, Munich and environs, and Berlin), we haven't seen the part of it I'm asking about. It would also get us within a short train ride of the Frankfurt Airport for our direct flight home (on points). One other factor: we might try to visit a friend who lives near Cologne.

I thought the Cologne Cathedral was a "must see," but maybe I'm naive. Lubitsch, if I'm reading you correctly, are you suggesting it is just a tourist trap? More so than St. Mark's in Venice, or St. Peter's in Vatican City or St. Paul's in London? Lots of tourists and trinkets there too. Boppard looks precious, thanks for that suggestion, Fussgaenger!

No, I meant Cochem with "tourist trap" not the cathedral in Cologne. The cathedral is very obviously a major sight in every respect. Though there are large and important cathedrals elsewhere but nowhere are there twelve Romanesque churches as in Cologne. And when I talk about tourist traps I don't mean lots of tourists or trinkets. I mean that places simply aren't worth it. Or they aren't worth the monstrous detours many plan in their itineraries.

But as I've already said the Rhine is long and your time is short. If you have just three days, that's very little time if we are talking all the way from Cologne to Mainz. You have to decide roughly between Cologne or Bonn or Koblenz and focus on one of the surrounding areas.

Friends to visit in Cologne area could make it a good stop as they can show you some neat places and there are plenty in that area and only an hour by high-speed train to Frankfurt.