Geocaching (http://en.wikipedia.org/wiki/Geocaching) has been around since May 3rd, 2000 and has turned into a popular activity for people who enjoy playing with GPS, hiking, offroading and just adventure in general.
I had heard about "letterboxing" before but this is something new (to me anyway) and it sounds like something fun.
There is even an edmtonon club.. I have been looking into it since I have a GPS now.
http://geocachingedmonton.com/