have one guy running frantically, being chased by a big gang of like 30+ people. eventually they'd catch him, swarm him and put a beating on him (fake of course). this would be good to do somewhere where there's a lot of open spaces, like golden gate park. you could also do the reverse- have one guy chasing the gang. eventually he'd catch them and put a beating on them, proving that safety in numbers thing wrong once and for all!

my other idea is just to have a mob congregate to starbucks and all use the same name. they wouldn't all come in the store at the same time and they wouldn't order the same thing - that would be too obvious and flash mobey. but i thought of it the other day when i was one of three mikes in a row.