Any museums in particular you liked?
The Pergamon Museum is stunning, do not fail to go there.
Also, go in the food hall at the KaDeWe department store. It is unbelieveable, and they have a good restaurant with great views from up there.
There is a good display at the remaining bit of the Berlin Wall, a kind of a museum and information centre.
Checkpoint Charlie is near there, interesting if a bit of a tourist magnet, and quite near the Reichstag there is the very large and recently-constructed memorial to the victims of the Nazis, together with an underground display and information.
That's worth visiting, particularly since you expressed an interest in that period / topic.
All of these places can be easily reached on foot, from each other.