Pillow Fight!!
This past Valentine's Day almost a thousand people convened on the Justin Herman Plaza near the Embarcadero for a gigantic pillow fight. (http://www.pillowfight.info/)
You can't tell from looking at these photos, but the air was full of feathers and fuzz, like a snowstorm of pillow innards, sticking to hair, clothing, shoes, etc. Apparently people stayed after it was finished and helped pick up the errant fuzz, as the plaza was pretty much feather-free at lunch the next day.
I went as a pillow-less observer, and it was pretty fun...