Whenever I'm returning a library book that I've read, I find a quote that has something to do with the book's subject matter and leave it on the first page of the book. That way, whoever's reading it next will have that to think about as they read it.
I leave "messages" various places. Sometimes I'll put biz cards inside the stalls at ladies restrooms, but usually I just leave one on a step or somewhere where someone would stumble upon it and think they should call the number.
I would put scatter local...