Welcome to FlyerTalk.

It take about two hours each way to travel by car between San Francisco and Monterey so I would plan on more time than one afternoon. There is a lot to see and do down there so you might consider spending a night or two in Monterey or Carmel.

Napa and Sonoma are both lovely and are easy day trips from San Francisco. Again, there's also plenty to do there to spend a few days. Some wineries may be pretentious; others aren't. There are far more that aren't.

Yes, I would definitely avoid driving in San Francisco. It's not the easiest place to drive or to find parking. If you wanted to go to Monterey though, I'd rent a car for that.

There are spring-fed spas all over the Napa Valley area. There are a lot of options there. The only ones I don't much care for are the sulfur springs spas - yuck.

This is all pretty general information. As you come up with more specific questions, I'm sure I and others will be happy to help.