So, my friend and I have a game that we have been playing since about 4 years ago - we started it at Comic-Con, and we're now going to try to play it at Wondercon.
The rules for the game are simple - first person to spot a guy dressed as Sailor Moon wins a free dinner. We're not talking about a steak dinner at Ruth Chris or anything, usually just, "Ahh, damn. Here's $10 for Jack in the Box."
2 years ago at Comic-Con, he and I both spotted a dude dressed as Sailor Moon (we saw him at the same time), as soon as his fiancé and my wife ran off to the bathroom. They didn't believe us, but dammit, photographic evidence worked against them!
You're more than welcome to modify this game for your own liking, but make it something kinda obscure - don't do "The Eleventh Doctor" because there's bound to be TONS of those guys running around. It's got to be something you don't expect to see, but you think you just might. Examples might be the Fake Stan Lee, or Morgan Spurlock, or Han Solo with cleavage (saw the last one last year - it was weird, but she was really good looking, it wasn't "hooker-quality", and she pulled it off).
I know I've seen websites with Comic-Con bingo, but they tend to make it really easy.