Now that the kids are out of school, I thought I would pass this along.
I saw a PBS show about this and thought it was so cool that I went out and bought a GPS handheld to do this with my kids: GPS Treasure Hunting.
Here is the crux of how this works:
Visit
http://www.geocaching.com/ and put in your Zip Code and a distance you are willing to travel and they will return a list of treasure hunt locations within your given radius.
Next step (you'll need a login) is to get the Longitude and Latitude corrdinates of the cache and the difficulty, etc.
Head out and when you reach the location (or about 20 to 30 feet within the spot) switch to looking for the cache by eye. This is where it gets exciting - once you find the cache (and if you have kids WOW are they gonna be psyched!) you can open it up and there are little treasures from previous hunters inside. Take one and leave one is often the rule. So be sure to bring along something to put into the cache. Typically the cache is something like a coffee can and the contents are kept in freezer ziplock bags to prevent moisture problems.
Generally there will also be a notebook for signing your name, etc.