What about a fake celebrity sighting.
Someone brings along a friend that has a good sense of hummer and doesn’t know anything about what where planing. Before hand you post their name and photo here and maybe a couple of things about them, i.e. What they do, what they like. Then everyone hangs out looking normal at the designated spot. If you see him before hand maybe look at him with that “do I know you?” expression.
Then some one asks “are you John Smith” followed by “oh my god its John Smith” then everyone starts running up to them screaming and asking for photo’s and autographs etc. Treat them like they are the most famous celebrity that ever lived. It would be a rush for the unsuspecting celebrity, and everybody walking around the place would leave asking “who the hell is John Smith?”. You’d want them to have an uncommon name so that latter if people Googled it they’d find stuff about this flash mob.
