
I found it online and we decided we'd start doing it as something cheap / free to do with our spare time as a 'couples activity' other than just cars, and started today.
Basically... There is an app / website, where people list co-ordinates for a container and you have to find it using a GPS and sometimes clues. Kind of like a treasure hunt?

www.geocaching.comGeocaching is a real-world, outdoor treasure hunting game using GPS-enabled devices. Participants navigate to a specific set of GPS coordinates and then attempt to find the geocache (container) hidden at that location.