Have you heard of this? Do any of you do it?
I recently read a magazine article (Nick Jr. or Family Fun probably) and it sounded very fun. I like hobbies with minimal expense that the whole family can enjoy and this seems to fit the bill. It seems that you create a rubber stamp for yourself or your family and have a journal for stamping in. People hide letterboxes with their stamp and journal in places like national and local parks and leave clues for finding it. When you find the box, you stamp their book with your stamp and your book with theirs and rehide it. It's modern day treasure hunting, and we've got several hidden near us!
Check out LetterboxingNorthAmerica for more information.