If you are sticking to day trips from a San Francisco home base, one good bet north is over the Golden Gate bridge then straight out to the coast (west). You're quickly away from most traffic and the scenery is terrific. Lots of different loops you can do in Marin and Santa Rosa counties.
South of the city, the coast (PCH) is beautiful but can be a real traffic jam, especially on weekends. There are plenty of terrific, generally lightly traveled roads in the hills between Foster City/San Carlos/Redwood City and the coast. Maybe head for Alice's Restaurant (I think it's in Woodside) and take off from there north, south or west. The back roads are great all the way to Santa Cruz.