Geocaching 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 or might not soon change a bit.
In the meantime, check out the badges we've earned so far:
(badges and belt from Project-GC)