I like the idea of more scavenger hunts but I agree with veraca; if you hide items, they run the risk of getting stolen.
However, I might recommend that you put up special posters or pictures around the con space (Hilton/Red Lion) that people can instead take a picture of and if they collect a certain number or all of them on their camera, they could win a prize. ^^
My one concern is that you might have to ask the hotel if it is okay to put them up for a day; that, and you might need to let con staffers know that these posters are NOT to be taken down - and it would be nice if they included a message in the booklet that the hunt is available for people to play, just to give you some exposure.