The Cook County forest preserves are littered with them, so more than once I've found myself seemingly out in the middle of nowhere covered in mud and scratches looking for a piece of paper to sign. So far I've found 17 and not found probably twice as many. I can't figure out how much of this is luck and how much is skill - but the thrill of the hunt and the feeling of victory when the elusive prey makes it fun.
I was really surprised to find out how many of these are hidden pretty much in plain sight in the middle of the city. The challenge with most of those is being able to retrieve the cache without anybody noticing.