Geocaching

Here's a fun activity for the whole family. Geocaching (pronounced - "geo-cashing") is an outdoor treasure-hunting game in which the participants use a Global Positioning System (GPS) receiver or other navigational techniques to hide and seek containers called "geocaches" or "caches" anywhere in the world. A typical cache is a small waterproof container containing a logbook and "treasure", usually toys or trinkets of little monetary value.
The official website that can answer all your questions is at:

http://www.geocaching.com/

The Rules - from: http://www.garmin.com/outdoor/geocaching/
(a maker of GPS units)
This growing outdoor sport has a simple set of rules, making it easy for anyone to play. The challenging part is finding the cache.
Once geocachers find a cache, they follow these basic rules:

1. Fill out the logbook
2. Take something out of the cache
3. Put something in the cache
4. Return the cache to the exact position and condition in which it was found

You never know what the founder or other visitors of the cache may have left for you to enjoy. Be sure to fill out the logbook with the date and time you visit the cache and leave something in return.