don't know how I didn't notice this one before. A few years ago, me, my boyfriend and some friends came across one by accident while out exploring at night. This year, he got me a gps unit with the intent of getting me into it, and it is pretty fun. Hell, it gets me out of the house and outside, and adds a "purpose" to walking or hiking, etc. A lot of them in urban areas are tiny - though some are still big enough to fit small papers/labels/etc. But out in the woods, a lot of the containers get a bit bigger, typically the size of ammo boxes or gallon tupperware containers.
There already a level of interest because it can get you out exploring places you didn't know existed, or discovering things that were right under your nose the whole time. It does make for a good atmosphere for including other little messages/whatever (though there is the emphasis on kid-friendly). They also have items meant to move from box to box "trackables", which could make for a good way to spread something to several people, rather than just one (not that either way is bad). It's a few bucks to buy an "official" one, but I'm sure there are unofficial ways to track as well.
At work, so not much more to say right now. But I'll be keeping my eye on this thread