Geohashing

 Geohashing /ˈdʒiːoʊˌhæʃɪŋ/ is an outdoor recreational activity inspired by the webcomic xkcd, in which participants have to reach a random location (chosen by a computer algorithm), prove their achievement by taking a picture of a Global Positioning System (GPS) receiver or another mobile device and then tell the story of their trip online. Proof based on non-electronic navigation is also acceptable.

Letterboxing

 Letterboxing is an outdoor hobby that combines elements of orienteering, art, and puzzle solving. Letterboxers hide small, weatherproof boxes in publicly accessible places (like parks) and distribute clues to finding the box in printed catalogs, on one of several web sites, or by word of mouth. Individual letterboxes contain a notebook and a rubber stamp, preferably hand carved or custom made.