In looking at the map, there are several options and I am looking for suggestions for things to see along the way. I will have about a day and a half to two days to make the trip.
I am interested in Cologne Cathedral and Heidelberg Castle, but they are on different routes. I'm open to other cities as well, such as Hamburg, Bremen, Hannover, etc. But with such a short window of time, I would like to maximize it.
Thanks for helping!