Geocaching is sort of like global hide-and-seek (with hints) for technology geeks.

I stumbled upon a non-GPS version of this diversion, known as Letterboxing (not to be confused with letterboxed movies, another subject entirely).

By the way, if geocaching isn't enough "fun with a GPS" for you, you can try The Degree Confluence Project. See the world with a GPS, experience other cultures, learn a few words of the local language:

The Thais kept asking how many farang (foreigners) did this and how much they got paid. I must have heard the phrase "farang Baar", (foreigners are crazy) more than a couple of times that evening.