To the letterboxing purists out there - what is on geocaching are letterbox HYBRIDS. Letterboxing predates geocaching (maybe the inspiration for it?) Letterboxing predates hand held GPS receivers, so navigation had to done by descriptions, hints, and the use of a compass at times. For more on pure letterboxing...
http://www.letterboxing.org/
Another non-GPSr activity is orienteering - navigating with just a map and compass - old school.