In one of my class, we had discussion about behavior change as the outcome of physical activity promotion and we ended up with a conclusion that changing the environment is the best way to achieve it, though not definitely the cheapest and easiest one. Then the next questions will be: What kind of environment do you need? What can you do to change it?
This video may give some hints
Hope I can produce more fun and creative idea than that funtheory.com! ;)