Long before the days of Edward Snowden and the NSA spy scandal, there was Teufelsberg, a man-made mountain in western Berlin known both for its views of the city and its unique position in history. At its top is an NSA listening post built by the United States during the Cold War for surveillance on Russia: long abandoned, the graffiti that covers it only serves to add to its mystique. Also of interest is the military school created by Third Reich leader, Albert Speer, now buried beneath the rubble of a mountain created from a decimated Berlin during post-WWII reconstruction.
There are plenty of hikes to be had through the Grunewald forest at its base to the summit of the mountain, although you'll be required to sneak through the barbed wire that surrounds the old complex. No matter; the sense of desolation and mystery at the NSA station (on top of some sweet photo ops) is enough to inspire the curiosity essential for a good adventure.