Geocaching, Earthcaching, and Other Treasure Hunts

Geocaching (including Geocaches, Earthcaches, and Benchmarks) https://www.geocaching.com

 

Letterboxing https://www.letterboxing.org/

 

Munzee https://www.munzee.com

 

Waymarking http://www.waymarking.com/

 

Wherigo https://www.wherigo.com/