A group called Improv Everywhere put a lectern with a megaphone and a sign that said "Say Something Nice" in New York City, allowing passersby to say whatever they wanted, and videotaped the results. The whole story with photos is here: Improv Everywhere.
Some people just walked by and looked, some took a photo of the lectern, one guy said literally "something nice", but a lot of people said "I love you", or "have a great day". Some people got more creative, and played guitar or sang.
I just love this. I don't watch the news because it's needlessly upsetting to me, but I do listen to NPR every day, and lately that has been just as upsetting. Sometimes I need a reminder that people are, in general, pretty neat.
(There is a short commercial before the video, sorry)