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Photo: Auto1234, CC BY-SA 3.0 via Wikimedia Commons
One of the few windmills in Berlin is located in Marzahn. After registration one can visit it and have a look at the functions of the windmill. To get there, take the S7 or S75 until "Springpfuhl" station, then the tram 8 or M18 until "Alt-Marzahn".
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Gärten der Welt
Inside you can find a large and well-established Chinese garden, a Korean garden, a small Bali garden/glasshouse, an Oriental garden with nice fountains and a cloister and a Japanese garden which is a project by the city partnership of Berlin and Tokyo. The latter has been built by Zen priests. Although quite crowded at times, there are not that many tourists so here's a chance to brush up on your German skills. As the journey to this park will be around an hour from the city center, don't miss this opportunity to complete your picture of Berlin by seeing some of Berlin's clean and quiet suburbs.
Before or after visiting the "Gardens of the World", tourists might be interested in climbing on top of the Kienberg (a small hill nearby with a view on the surrounding suburbs) or to go for a walk in the valley of the Wuhle (a small river). More information on the area can be found on the district's official website (in German).
A small castle in late classical style. It was built 1868 by Martin Gropius (uncle of the Bauhaus-founder and other architects, the von Siemens family changed the castle a bit around 1900 and they enlarged the dimensions of the park, which is today renovated and nice to wander around when the sun is shining. Located within a few minutes' walking distance from Biesdorf station (take the S5 from the city centre) or Elsterwerdaer Platz station (U5).
Stasi Prison
Experience the Stasi Secret Police Prison first-hand. Tours are compulsory. Some of the tours are done by former inmates.
Friedrichsfelde Central Cemetery
The cemetery is also known as the Memorial to the Socialists. It has many interesting monuments and graves of known people.