This is a great idea for urban fitness training. It’s called “Parkour” and seems to be more fun than conventional jogging or riding a bike. The aim of parkour is to condition your body to move quickly by using the environment around you. As defined in Wikipedia Parkour is:

a physical discipline of French origin. It is an art form of human movement, focusing on uninterrupted, efficient forward motion over, under, around and through obstacles (both man-made and natural) in one’s environment. Such movement may come in the form of running, jumping, climbing and other more complex techniques. The goal of practicing le parkour is to be able to adapt one’s movement to any given scenario so that any obstacle can be overcome with the human body’s abilities. 

There is a lot more of information on parkout at the UK Parkour Association website and this article.
Cost:  0$ for urban training and conditioning your body for real situations.