Physical (demanding) activities in Berlin

We're six guys going to Berlin in July on our yearly trip. We've been there seven times before, and we're looking to do something physically fun.

We've previously:

• played a lot of soccer / basketball in one of the many outdoor courts;
• climbed on Mount Mitte (great fun!);
• gone canoeing on the Spree.

We're looking to do something different and fun. Last year we did Mount Mitte and that was great.

We're physically somewhat fit, and we can communicate fine in German. Ideally, we want something which costs less than €100 per person, so that rules out skydiving.