Berlin, on of those cities I will happily live in if I had to choose something outside of the Netherlands. We went on a trip to the Grünwald, a big forrest on the outskirts of Berlin. There you have the Teufelsberg (Devils mountain), an artificial made mountain on top of an old SS training centre. That mountain is made out of the Berlin rubble from the 2nd WW. On top of that is an old NSA spy centre that was active during the cold war to spy on the East. Now it is a creative spot with a lot of street art!
Go and have a look.