Perhaps this is juvenile, but I think it would be amusing if part of the meetup consisted of zealously handing out "The Twelve Virtues of Rationality" pamphlets (which conveniently is available in pamphlet form: http://yudkowsky.net/assets/pdf/twelve_virtues.pdf); like you see Christians handing out evangelistic paraphernalia in shopping malls, street corners etc. Ideally in some sort of uniform dress, like monastic (Bayesian master inspired: http://lesswrong.com/static/imported/2008/03/27/elimonk2darker.jpg) cloaks.

Or if you could integrate the pamphlet distribution as part of a flash mob activity/public prank.