There is something loads of people do thats not a million miles away from this called geocaching. People stick stuff in various boxes including a notepad and pen, post the coords or puzzles to get the coords on the internet then people sign up on the internet that they have found the thing - generally with some proof, like a picture or the date of a coin in the box etc.
gf is obsessed with it, personally I find it the best way to ruin a good walk.