Back when I was in high school and early college, there were these things floating around the internet called letterboxes. You went to someone's amateur GeoCities webpage and followed a list of instructions (go to X place and walk 5 paces to the north then turn left, etc) in order to find a hidden box with a piece of paper and pencil or a stamp, and occasionally a treat. Some people got very elaborate and had whole trails of hidden boxes that you could follow even across state lines, each box containing instructions to the next's whereabouts. Some had travel buddies, a stuffed animal or figurine that, if you found it, you were to take with you to the next location and leave it there for the next person to find the box.
Nowadays with the ubiquitous GPS, some enterprising bloke has decided to market this 150+ year old pasttime into the geocache. Geocaching seems, to me, immensely more popular than letterboxing this side of the Atlantic, and if you do a search for your location on the website, you are sure to find a great many coordinates of caches. I am surrounded on all sides. There is even one hidden in the back parking lot of my place of work that I have failed to locate on three separate occasions while being stared at and questioned by confounded coworkers.
Geocaching (or letterboxing) is fun to do when you are bored, feel like exploring new surroundings, or merely want to go on an adventure. It helps to have friends to help you search as some caches are hidden quite cleverly and the coordinates are often slightly less than precise. After searching for four geocaches, I have successfully found two with my roommate Kimmy, who was an avid geocacher in California.
Evidently, you are supposed to be sneaky while searching for your intended cache, and those who are not searching - innocent passersby, curious store employees - are referred to as "muggles," a term ridiculously stolen from Harry Potter, though it was added to the Oxford English Dictionary in 2003 for a person who has no skills or is without knowledge. Ignorant, basically. (Though I think just to annoy people, I'll pretend it carries its 1920s slang meaning of a certain kind of illegal cigarette.)
If you haven't heard of geocaching, it's interesting to look into. Who doesn't like a good treasure hunt, after all? Not quite as mystical as fairy doors, but still an enjoyable pasttime.