I used to be really into penpalling and we would exchange friendship books. I had one friend that was into letterboxing, but I was too young at the time to be able to travel anywhere to participate. Now as an adult, I love the idea and wonder if anyone else does this as a hobby.
For more information:
http://www.letterboxing.org/
I'm going to do a few letterboxes in my area because there aren't any. It seems to be a lot of fun, if you like "treasure hunts" and hiking.

Sara