Notice how people not only walked up the stairs, but many of them added extra steps and even jumped around? How much wonderful exercise did this little experiment add?
TheFunTheory.com includes several other examples of making good behavior fun. I'm afraid the pleasure would be so short-lived that the fun would soon disappear. For example, the "deepest bin," designed to encourage people to throw trash where it belongs rather than on the ground:
Or the bottle bank arcade:
However, some other ideas, I think would offer long-lasting rewards. For example, the speed camera lottery:
What are you aware of that you could do that would make something truly good and useful--but less than compelling--to be more fun