So you have blocked off one day of your San Francisco trip to explore outside the city, where do you want to go?   Here are a few ideas of places you can get to (with and without a car).

Sausalito- Just across the bay with wonderful views of San Francisco and the Golden Gate Bridge. You don't need a car, the ferry drops you off near the main street in town.  You can walk around and browse the boutiques and shops. Consider going over on a Sunday morning around 11:00 am and going to "Horizons" for brunch.  They have a patio in the back that is on the water.  When the weather is warm it is a perfect place to have a nice meal and look at the fabulous views of the city.  Many people rent bikes in SF and bring them on the ferry to ride around in Sausalito. It's a perfect little getaway. The Sausalito Chamber of Commerce offers a detailed list of attractions and tours.

Muir Woods- Redwood Forest- Take your car or arrange to go with one of the many tour companies. Go see the historic Redwood Forest. These historic trees are really a sight to see.  The park offers many different hiking trails with varying degrees of difficulty. Useful information from the National Park Service about Muir Woods: http://www.nps.gov/muwo/index.htm

Tiburon- It is also on the other side of the bay from San Francisco and has a huge waterfront park that overlooks the bay.  It has many cute shops and waterfront restaurants. More information about this area on: http://www.landmarks-society.org/

San Jose- This area is not too far away by car from SF.  You can take a day trip down to see some of the museums.  Do not miss the Winchester Mystery House.  It is cheesy, but it really is a sight to be seen.  This is the house that Sarah Winchester had built and they continued construction on it 24 hours a day until her death.  

On the way home from San Jose you can stop by Half Moon Bay or one of the other beach towns.

Napa, Sonoma County - You can get tours that will take you to the wineries (probably the best way to go if you plan on lots of "tasting").  The trip to Napa can be a little long; Sonoma is a little bit closer.  Most wineries are open for tastings and have plenty of wine and other  delicacies you can purchase.