I don't really know how this would work but it seems like there could be fun involved.
This is a topic that has much interest.
Project Hidden Treasure : Bitcoin Promotion Thru Geocaching -
http://bitcointalk.org/index.php?topic=50166.0Geocaching for bitcoins -
http://bitcointalk.org/index.php?topic=69188.0And just a few days ago:
Today I tossed some Bitcoin, about a quarter ($.25 USD), into a fountain.
I'm thinking there could be some Guerrilla marketing type of opportunities as well.