Yea...I like the geocaching thing too.
I accidentally found a geocache 2 months ago. I was sitting at my fav spot down by the train tracks having a smoke and I saw this coffee can hiding behind a rock. I opened it up and it was someone's geocache.
It had a pencil and a little book you could sign if you found it. In the little book, you could write down your name, what you took from the can, and what you left in the can. I didn't have a lot on me at the time, so I left a pack of tic-tacs in the can.
But, I think that leaving a flash drive with some BTC and instructions would be a very cool idea. Posting on the geocache website would defiantly bring a new audience to bitcoins. Leave your email address in case the finder needs help understanding what a bitcoin is or if they need help importing the wallet file.
Dam...I might even do this myself! but, I need to get a handheld GPS unit first I guess.