Letterboxing is a hobby that had its roots in England in the mid-1800s. In its modern configuration, the participant carves (or purchases) a rubber stamp which says something about herself, and obtains a small notebook and a stamp pad. Using clues available on the internet and from various mailing lists, she heads out to find the letterbox.
Once found, she opens the box where she'll find another rubber stamp and another small book. She stamps the box's stamp into her book, stamps her stamp into the box's book -- and replaces everything as she found it, as surreptitiously as possible.
Why do this? It's fun! It also teaches map-reading, navigation, reasoning skills, some crafts the girls may not have ever tried, and gets them outside walking. This activity is much more acceptable to many young women than the more athletic-sounding "hiking"!
The fun is in finding the letterbox, finding out what the stamp in that box looks like, adding its impression to your collection -- and learning that you too, craft-challenged or not, can produce a pretty good rubber stamp for yourself!
At a volunteer retreat in the mountains south of Palm Springs, I taught a large group of adults to make simple stamps using "fun foam" as the stamping medium and wood scraps as the support. The procedure is described in my Easy Personal Stamp-Making Page. For a nice explanation of the "right" way to do it, see Carving 101 or any of the sites listed at Silent Doug's Letterboxing Site -- Stamps, Carving, and Bookbinding.
Probably the best place to start is at the Letterboxing - North America site. There are many very useful links on this site, including a number pertaining to stamp carving, the history of letterboxing, and -- most important -- a large repository of clues. (There are probably a number close to here you live!)
Another site with an excellent collection of clues is Atlas Quest, which organizes its clues a little differently (most boxes on one are on the other, but not all...)
In a word -- yes. I've planted
See also Silent Doug's Adventures In Letterboxing -- a Letterboxing workshop he put on for Penn Laurel Girl Scout Council.