Located in the Grunewald Forest the abandoned Spy Station has great views of Berlin, some amazing street art and interesting history. Teufelsberg is one of the most unique spots in Berlin. The spy station was made in the 20 years following World War 2 on top of 75 million tonnes of rubble moved from Berlin.
The forest is also great for hiking. The Spy station has some amazing photo opportunities, so its great for photographers. Also check out the huge Sand dune near by called the 'NSG Sandgrube'. Take the S9/ S75 to Heerstraße, or S1 to Grunewald and walk through the forest from there.