Moveabout Theater (http://www.moveabouttheatresf.org/upcoming-events.html) is doing a free production of "The Importance of Being Earnest" in the park on the hill above Fort Mason. Because it's outdoors, the "offstage" actors will be standing still at the edge of the action, so should be good drawing subjects. The cast is all San Francisco high school students and they'll be wearing costumes from the 50's. And if you don't want to sketch people, there are plenty of great views over the bay and city from here.
After the show ends we'll meet up at Cafe Franco in the SF Hostel, at the north end of the Green overlooking the bay, to share sketches and wrap our hands around some hot drinks to warm up. In this sketch of the community garden behind the Green (http://www.flickr.com/photos/55162025@N00/8114323459/in/set-72157629622643385), the Hostel is off to the left.
Something to sit on and warm clothes--it can get really windy here.
Bus: The 28 goes right to the base of the hill, but you can walk 3 or 4 blocks from the 19, 22, 30, 47, 49 buses.
Bike: Nice place for an extended bike ride along the bay.
Car: I think you have to pay now in the main Fort Mason lot, but there's plenty of free parking a little to the west, along the marina.