Thanks guys, found a couple of things to add to the list of possibilities:

Michael Schumacher Kart centre:
http://www.ms-kartcenter.de/en/home.html

Nuremburg ring go karts:
http://www.nuerburgring.de/en/drives.../kartbahn.html

Paintball:
http://playpaintball.de/paintball-spielen/


I also found a couple of model shops around Bonn / Cologne, however as there are a heap of model shops at JP, it kind of doesn't make sense to go looking for more..