Good shout, follow the Rhine before getting to Koblenz (A61) and head down into the Rhine valley for any of the smaller villages which are spectacular with their small castles over looking the vineyards and river. Before that you have the Black Forest which is great although it will be bloody cold at this time of year but city wise there is Baden Baden.Would go via the Moselle valley for the vineyards and to Koblenz and then Cologne.
Avoid Karlsruhe, dirty industrial city and I found Bonn a bit bland.