In Yunmeng Xianjing Natural Scenic Area, you can get into some white water rafting. For something a little more extreme, check out flying man paragliding club. This paragliding group will teach you to fly just outside of Beijing in Changping.

Depends on what your definition of 'extreme' is, but Mash Up Asia hold a host of different sports including basketball, softball and flag football etc.
Not your average white-knuckle, adrenaline-pumping thrill sports, but to be honest most of the guys who turn up are massively hungover, so it feels pretty 'extreme' when we play. In fact it's an effort not to throw up on the pitch.

Check the link below for more info.

There are loads of bungee jumping sites around Beijing. Check out these articles for more info:

http://www.echinacities.com/beijing/city-life/bungee-jumping-in-beijing-the-8-best-sites-part-1.html

http://www.echinacities.com/beijing/city-life/bungee-jumping-in-beijing-the-8-best-sites-part-2.html

Try the subway at peak hour.

There was an extreme dirt bike competition in Beijing not to long ago. It was televised and the area they had the competition was the Olympic Stadium area. Most of the extreme sports will be in that general area around the Olympic Stadium. They had people competing from all different countries including America, Australia and China. The American extreme dirt bike rider won I watched the whole thing. 

Crossing the road?

Driving?
Competitive breathing??
Scam avoidance?