There are so many great websites out there that focus on sending mail to strangers or receiving mail from strangers. I Got An Envelope is one of my favorites. The basic idea: You leave a self-addressed envelope in some public space with instructions that the finder put whatever he or she wants in the envelope. The finder sends the envelope back to you, you photograph the contents, and then you send them to the I Got An Envelope site. I left an envelope once, but nobody ever sent it back to me.