Another good cruise, to or from Nuremberg which is three hours from Wernigerode, is the Main-Danube cruise....say from Nuremburg to
Budapest or vice versa (there are a lot of variations). The Main-Danube canal over the mountains that seperate the Danube basin from the Rhein basin is a spectacular bit of civil engineering with lock entrances that look like tunnels into the mountain, and so on. And you visit some pretty neat places along the way.
But a big advantage of doing the HSB before or after a river cruise is that you can do better than be a day tripper. You can spend a couple of days and do it right. As Michael pointed out, there is a lot of tourist interest in and around Wernigerode so the wife does not have to feel like she is on a railfan trip. And the Harz Mountains (actually more like rolling hills) are very pleasant.
JBWX