Going to Germany

OK, I'm going to be for few days in Germany, more exactly I'm there to see the Roger Waters's concert in Hamburg.
I'm gonna spend one day in Hamburg and one in Koln, that's not too much time...
So, considering the short time, can anyone tell me what is there really worth seeing?
I mean, not night life or restaurants and things like that, but something significant, something unique perhaps?
For example one thing that I will definitely visit in Koln is the "Cologne Cathedral", an extraordinary building that always fascinated me, and now is my chance to actually see it.
I know that Hamburg is one beautiful city and there is plenty to see there and I already found something to my (maybe dubious?) taste: Miniatur Wunderland "The world's largest model railway museum" Yeah, don't laugh, I like toy trains
. But what else...?
Yeah, I know about Wikipedia and travel guides and such, (I once read a travel guide about my city and was almost nothing there that I would recommend visiting.) but I prefer to hear some real people opinions.
I'm gonna spend one day in Hamburg and one in Koln, that's not too much time...
So, considering the short time, can anyone tell me what is there really worth seeing?
I mean, not night life or restaurants and things like that, but something significant, something unique perhaps?
For example one thing that I will definitely visit in Koln is the "Cologne Cathedral", an extraordinary building that always fascinated me, and now is my chance to actually see it.
I know that Hamburg is one beautiful city and there is plenty to see there and I already found something to my (maybe dubious?) taste: Miniatur Wunderland "The world's largest model railway museum" Yeah, don't laugh, I like toy trains

Yeah, I know about Wikipedia and travel guides and such, (I once read a travel guide about my city and was almost nothing there that I would recommend visiting.) but I prefer to hear some real people opinions.