The Gasometer Oberhausen is a former gas holder in Oberhausen, which has been converted into an exhibition space. It has hosted several large scale exhibitions, including one by Christo and Jeanne-Claude, which is going till the end of december 2013. The Gasometer is an industrial landmark, and an anchor point of the European Route of Industrial Heritage and the Industrial Heritage Trail. It was built in the 1920s, but largely reconstructed after World War II.
I was there this summer, when I did a bicycle tour through the Ruhrpot. It is huge, I can tell! I was at the big air package, the Christo and Jeanne-Claude exhibition. Next to it there is the tree2tree climbing area, I guess in winter they’re closed but I was there a few years ago and it was a lot of fun!  Also not far away there is the Rhein-Herne-Kanal where you can swim in summer (that’s also a lot of fun ; )

for more infos – check here:
http://www.gasometer.de/en/exhibitions/upcoming-exhibitions

And next to it there is a tree climbing area!
http://www.tree2tree.de/de/hochseilgarten-oberhausen.html

for the NRW ticket – check here
Free trains in NRW