Geocaching.

http://www.geocaching.com/

Global activity (even under water).

Entertain.

Everyone. Some places might be hard to visit for people with some kind of disability.

Geocaching 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 value.

*information gathered from wikipedia page about geocaching.

For centuries, the hunt for hidden treasures is seen as a big adventure for which you had to be an adventurous traveller or pirate. Now everybody gets a chance to feel what it's like in small or larger journeys. By leaving something in the treasure chest that belongs to you, for each person who visits next, the discovering of the treasure will be a more fun and interesting experience.

Standard gps receivers are good enough to play. Someone has to let you know where to look, so an internetpage with coördinates is usefull.

Pure gps navigation. It's up to the player which device or software he uses.

Insert gps coördinates found on the internet. Walk/run/drive to this point and return back home or continue to the next point.