Geocaching (official site) is a fun outdoor hobby/activity, in which you use GPS coordinates to locate secret containers hidden all around the world. Some of these "caches" are simple to find, others hard. Some are located in urban areas, others in nature, some types give you the coords from the get-go, while with others you need to first solve a puzzle or visit multiple locations. They range in size from tiny magnet caps barely able to hold the logbook that proves your find, all the way to large chests full of space to trade knick-knacks and personal collectables bearing the usernames and designs of the individuals and teams participating. It's also a hobby that's remarkably easy to get into (these days, all you really need is a smartphone and a pen), which to me balances its more "hidden" nature.
I haven't been doing much Geocaching as of late, but that might be slowly changing.
Geocaching has always involved the community to a large degree: all caches are created and maintained by players, they organize and attend meetings called "events", and there are even entire aspects of the game created by them. One of these is what I've heard referred to as "Personal Signature Items": instead of using the caches to drop off and trade random small items, some people prefer comissioning souvenirs bearing their username and/or logo, and exchanging these instead. When it comes to standardized signature items, we Czechs (along with some other European countries) tend to use Czech Wood Geocoins (CWGs, not to be confused with regular Geocoins, which, like Travel Bugs, are meant to travel cache to cache, player to player) – wooden, laser-engraved tokens. What they lack in color, they tend to gain in cheapness, and as such, the amount you can afford. Our friends over in the US usually go for Pathtags – smaller metal tokens, hand-painted, arguably much more pretty, but also more expensive.
Another of these "unofficial" aspects are digital badges. Unlike Groundspeak's official "souvenirs", which are usually awarded for limited time only as an incentive to play more, these are awarded in tiers based on trackable accomplishments. Number of large caches found, progress through challenges, most types of cache found in single day... They are generated out of your numbers in, what I can only describe as, the statistics-focused fan project Project-GC. If you would allow me to "brag" a bit, here are the ones we have earned so far: :)
Finally, I should mention games similar and alternative to Geocaching. There's the open-source alternative Opencaching, and mostly American players can run into Letterboxing (not to be confused with the Letterbox Hybrid, a cache type specifically designed to bridge the gap between the two).