Some time ago I was introduced to Bookcrossing . This is a really fun activity that you can do as a family from time to time. It is almost like a scavenger hunt for books.
If you log into Bookcrossing.com, you can find out how to become a bookcrosser. It is simple. You log in a book you want to release into the "wild". You print out a label for it, apply it to the book, then release the book. Who knows where the book will go. The idea is that another reader will find it, and log it in to the site, read it and eventually re-release it, allowing the book to travel to another place. Also just as fun is the hope that one day you will find a traveling book.
I have logged in several books and released them (you can view my bookshelf ). I have only had a couple "found". The best luck I have had is in actually handing the books to other readers, asking them to log the book in, read it and someday release it. Other successful releases for me have been taking them to an "official crossing zone". This is a box that stays set up at a local community college where books are rotated frequently.
It is fun, and a little sneaky feeling to leave a book laying around, just conspicuously enough to hope another reader will find it.