Although it is not exactly a secret spot anymore, the Spreepark is one of the most magic places in Berlin. It’s mystic, it’s abandoned and it’s most certainly a piece of old Eastern Berlin. You should visit some time soon and bring your camera – because rumour has it that the city-state is looking to get hold of the area and turn it into an appartment block.
For background reads on the story of the amusement park and the family who runs it:
The Story of the Berlin Spreepark (English)
Spreepark in Berlin: Mit Vergnügen ins Desaster
If you are looking to get in you should give this a read: