My first thought on seeing Monday's XKCD was that would likely be an interesting twist on geocaching or location-based games.
Speaking of geocaching... I have started letterboxing, which is somewhat similar. It is a lot of fun. :)
Copyright ©2000-2008 Jeremy Mooney (jeremy-at-qux-dot-net)
A friend's dad told me once that they used to play a game like that but using radio. One car would take this antenna type thing (described as a loop) and the other car would have a detector that would get a higher pitch when it was aimed in the right direction. That would be a fun game even today.