I'm glad this forum is here, this should lead to some great stories!
I once hung a book from a tree in a clear ziplock bag, it was on World Peace Day or similar (I forget the details) and I released a book on meditation because it seemed appropriate. It was in my favourite park near my home, and when I walked back an hour or two later it had gone. The next day, someone had left a journal entry thanking me very much - someone had broken into their car and they were having a not-so-good day, and there was this book on meditation hanging from a tree.
Made my day. :D Random acts of kindness FTW!