Get a megaphone, and call out to everyone. "Hey everyone, can I have your attention for a moment?" and when they settle down and look at you, pause for a moment as they stare quietly then say "Ok, thanks for your attention." and then walk away.
Alternately you can call out "Hey Stupid!" and see who looks.
we had a con on april fools, and I had an evil idea that I would never do but I'll share it though :v I thought an evil prank would be to give cookies to everyone (maybe as part of a cosplay) but have laxatives in the cookies. imagine a cosplayer with a costume that takes a long time to get out of having an instant case of the poopers. again, this prank is pure evil and I wouldn't attempt it :v
One of my evil pranks I would love to pull but can't would be messing with the one person on dA I hate more than anyone else. She blurted out to everyone on Tumblr that she hates coffee and is afraid of it. My friend and I thought about getting her a mocha and showing up to a convention(she doesn't know what either of us look like) and offering it to her while telling her its hot chocolate.
That has to be the most evil prank I've ever thought up and while its pretty insane, I obviously can't pull it off. It would be pretty funny to see her horror face when she realizes she's drinking coffee though XD.