I have, for several years, participated of the 24 pull requests challenge. It has been an overall very positive experience that has let me to meet people and contribute to projects that I'd have otherwise not been able to do.
Now, an interesting idea came about working on the Mamook Lapeep project: allow people to offer immediate actions in their local communities as a "trade" for the art piece. This idea is an extension of that, a way of tracking good deeds as a "challenge" similar to 24 pull requests.