It reminds me of Geohashing, a similar sport to Geocaching >>
http://wiki.xkcd.com/geohashing/Main_PageGeohashing is a method for finding an effectively random location nearby and visiting it: a Spontaneous Adventure Generator. Every day, the algorithm generates a new set of coordinates for each 1°×1° latitude/longitude zone (known as a graticule) in the world. The coordinates can be anywhere — in the forest, in a city, on a mountain, or even in the middle of a lake! Everyone in a given region gets the same set of coordinates relative to their graticule.
Geohashing looks fun
There are lots of ways to potentially set something up. In that treasure hunt game, as I recall, most everything - gameplay, riddles, etc. - was contained in the game, and you could talk with others online. However, I do think there may have been some real world components too. For example, extra clues might reference a structure in front of some museum, or maybe something found in the largest library in Los Angeles. Teams formed with an understanding to split the prize. It got to be a pretty popular thing.
Thanks to all who commented so far. Lots of idea were thrown around, from "don't waste your time" to puzzles to geocashing. I'll get to it this weekend, most likely with a simple QR code, brief explanations, and pointers to a few Web sites.
I liked your original idea, but thought it fairly small scale, with marginal effectiveness. However, you may have given inspiration for something much larger. You (or anyone) can of course continue taking any actions you want, but I'm thinking we collectively might also explore a larger game based project, and maybe pool a prize of several thousand dollars to add some kick.