My friend Kristen has been doing this for awhile and last night she took me out to try it. And it's just as wonderful as it sounded!
Letterboxing is basically a treasure hunt where you follow clues (on a centralized letterboxing website) to find a box hidden somewhere, in cities, parks, wherever. In the box is a hand-carved rubber stamp, usually with an image that represents the place where it's hidden, or sometimes representing the person who made it.
In the box is also a little blank book (often handmade also) which is a record of everyone who has found the box. You stamp your own personal carved stamp in it and put the date you were there and your name. When you find the box, you get to see all these amazing stamps from all the people who have been there, with the dates they were there and their name. You take the stamp in the box and stamp it into your own little book where you collect all the stamps from the boxes you've found as your record. awesome! Kristen says letterboxing has been around since the 1800s, when they used to hand carve stamps and hide them, and clues
Then you go to the letterboxing community website and log your finds, print out clues to lead you to more, etc.
One etiquette item about letterboxing is to never show photos of the stamps you find or basically what's in the boxes you find - so as not to spoil the surprise for everyone that goes looking for it after you. So, here are photos of our adventures last night finding three boxes around central Austin. See the rest of the photos at my flickr page.
These are the stamps I brought along to stamp into the books. Some of the boxes and therefore the books are so tiny that the big Niku stamp didn't fit. I'm going to design and carve a new stamp to go along with my "trail name", Niku Sweet Tooth :) and the little book I brought to put the letterbox stamps into.