In a nutshell, I've been leaving notes that say "Smile At A Stranger Today" on the NYC subway. I leave them on the seats, taped to the walls, anywhere I can. Everyone is so serious these days, I'm just trying to lighten things up and bring some smiles back into it!
Maybe you'd like to make a note and hang it in a public place? It doesn't have to be on the subway--it could be on a lamppost, at the mall, in a restroom. Or maybe you want to hang one on your mirror, you know, just as a reminder to yourself!