Can offer up some suggestions for Berlin, most of my interest is in the post war period.
Stasi museum at their old HQ is very interesting as is the Palace of Tears at Friedrichstrasse
Walking tours that start from outside the McDonalds at the Zoogarten station are an excellent way to see the city
Several topics to choose from, Google insider Berlin tours
Brandenburg Gate and Reichstag obviously, river boat trips seem popular too.
Checkpoint Charlie is very touristy, last bit of the wall is worth seeing for historical reasons, but not much left of it
Other smaller attractions such as the East German motorcycle museum at Alexanderplatz are worth searching out
Gaslit street lights in the Tiergarten make for nice night photo subjects, to be honest its a big city and too much to deal with here