SCVNGR

I am going to be trying something different every week, and maybe every day because as I do different things I improve in some way. I've been toying with the idea of using some sort of game or challenge to get people motivated to join me on the subway. Specifically I want people to hold up colored construction paper showing me and everyone else on board that they like what I'm doing. I discovered SCVNGR which is a platform to create and perform real world challenges for real world rewards (like a free cup of coffee). So I am trying to think how I can use this to challenge people to take action and do something that helps get this subway community going. If any of you can think of what exact challenge I should create on SCVNGR and what reward to offer, please let me know. I plan on handing out fliers on the train this week describing this problem to see if anyone can help.