Where would you stop on the way from Amsterdam to Berlin? Maybe Bremen?? I'm open to suggestions! Maybe a change of itinerary?
Two nice places, more or less on the way, small, quaint towns, would be Wernigerode or Quedlinburg, on the north side of the Harz National Park.
Dresden is not really on the way from Amsterdam to Berlin, is a good stop between Berlin and Prague though.
Wernigerode and Quedlinburg look really nice!
Has anyone been to Erfurt?
Hammburg has quite a bit to see but it is a large city. You may prefer something smaller like Hildesheim or Hamelin. I've never taken the train from A'dam to Berlin so I'm not sure what stops are convenient on your route.