Geocaching is a sport in which you try to locate a specific destination using a GPS receiver.

The destination is known as a cache.
The cache will be identified by clues and a longitude/latitude reference.  There are varying degrees of difficulty in both terrain to cover and challenge of locating the cache.   Normally, the clues are difficult to interpret, adding to the challenge. 

When you succeed in locating the cache, you submit you success on the Geocaching web site which tracks how many you have "bagged".  In some cases, there is an actual container at the cache in which you write your name to prove you found it.  In others, your arrival at the cache will be verified by answering some questions based upon things at the destination.  More complicated cache hints will involve a series of locations, each of which is only found when the previous one has been located and new clues are interpreted.

Here are a few local caches:
The Real Price Of Freedom (an easy one to start, although the actual cache is hard to see)
UTB Cache (easy to find the location, but the actual cache is tough to see)
Out Of The Blue (a fairly easy one in a graveyard with specific questions to prove you were there)
A Stroll Through The Statues (a complex one that runs you from place to place)

Go to the Geocaching web site and search by zip code to see others.