Typical cache treasures aren't especially high in intrinsic value, but often make interesting souvenirs: $2 bills or other unusual coins or currency, kids' toys from fast-food restaurants, ornamental buttons.
There are several variations on the basic geocaching theme. Some caches require the seeker to solve a puzzle or visit one or more intermediate points to determine the coordinates of the actual cache. Some are "microcaches", too small to hold a logbook. A very few are virtual caches, with no container at all, usually at a site which is interesting in and of itself.
Geocaching is growing rapidly in popularity all over the world.
Links:
See also: letterboxing
wikipedia.org dumped 2003-03-17 with terodump