Some sharp local prankster is organizing a Bay Area chapter of Improv Everywhere's annual No Pants! Subway Ride. The New York based-instigators encourage cities around the country to start up their own naked-kneed chapters, catching commuters off guard by catching trains with their bottom halves bare. If you're down, you can read all the rules for the SF crew here (short version: get on the Pittsburgh/Bay Point train at the allotted place and time, which lasts between 1:30 and 2:30 p.m.). There seem to be a couple different organizers, though, each with their own ideas of where to meet (this one's a little more general, this one more specific). Between the three, I'd say that first local mentioned seemed to have their shit together most, an important factor if you plan to allow this stranger to lead you into public half-nakedness.
A rich history of No Pants! Subway Rides here.