I love Improv Everywhere, the New York-based "prank collective" that stages interesting things all over the city. Here's a link to a mission with a simple premise: two actors placed a megaphone and lectern in Union Square. Mounted on the lectern was a plaque that said: Say Something Nice.
Almost everyone who picked up the megaphone complied with that simple instruction. You can read about the mision at the link, but here's a sampling of responses:
Say something nice today, folks.