Gadling came through with another great article today. The Guardian has posted on 20 Urban Adventures that pretty much anyone can do in great cities all over the world.
The ones that get me going include: Ice Climbing in London, Caving in Budapest, and Adventure Racing in Edinbrugh. There is pretty much something on the list for everyone, and even if you can't get to one of these cities, perhaps it will inspire you to find something similar to do in your own city.