Ok, so what is letterboxing? Basically it's like a treasure hunt. People "plant" letterboxes at public places such as parks, bike trails etc. and then they post clues about where they are located online. To find out more about letterboxing and find clues online go to http://www.atlasquest.com/ and/or http://www.letterboxing.org/
The boxes are usually small tupperware/plastic containers that seal tightly or ziplock bags. Inside a letterbox you will find the rubber stamp of the person who hid the letterbox and usually a little log book, journal where you the finder can leave your own stamp, trail name and date you found the letterbox. You will be able to see who has found it before you as well. You should bring your own journal or paper to collect the different stamp images and then you can create a scrapbook or whatever with stamp finds :)
When you are done stamping you need to put everything back into the plastic container, seal it tightly and place it back where you found it, making sure it is hidden. You must also be discreet when doing this, so non-letterboxing people don't notice you and come and maybe take/move or destroy the letterbox.
So those are the basics of letterboxing - I've read that many letterboxers make their own custom hand carved stamps, so they are totally unique.. We plan to plant some of our own letterboxes once we have found some more and get a better feel for letterboxing...
This past week we went on two letterboxing quests - I will post about those next ~