How to celebrate a birthday in Berlin in an unusual way?

[einermeiner]

This year I want to do something out of the ordinary, not just sit in a bar. I think to gather friends and make the holiday more active, perhaps with a small route around the city. What are some interesting options where you can combine fun, socializing and something unusual?

[einermeiner]

How is it organized? Is the route fixed or can you choose your own? I wonder how long the whole trip takes and are there any restrictions?