Over the last five or six years, when I've rented a book from the library that I've really liked, I've drawn a simple, b&w bookmark for it, and left it in the book.
There's not a lot of them out there, but at least five or six now. I have no idea who will see them next, or what happens to them. I usually add a note on the back to please leave them in the book, but who knows?
It's just fun for me. Plus I have a nice, themed bookmark while I read it, til I return it.
So, have any of you ever done this? No? Inspired to do it?