Sketch old cable cars and big wheels turning. It's all indoors, so rain and cold will not be a problem. The museum is free, but donations to this wonderful old San Francisco institution are much appreciated.
3:00 Meet up afterwards at the Gallery Cafe across the street to share sketches and chat.
Bring a folding stool if you have one, as there might not be a good place to sit near the thing you want to sketch.
Public transportation is the easiest way of getting there. From the Powell Street BART/MUNI station, you can catch the Powell Street cable car (doh). If you want to get there a little faster, go to the Montgomery Street station and catch the #30 or 45 bus (get on at 3rd and Market or on Kearny). Bus lines #1, 10 and 12 will also get you close.
I don't know anything about the parking situation in that area. If you have any tips, please post something in the comments section below.