My partner and I will be visiting San Francisco for a week in early May. We would like to make a short trip to Napa or Sonoma valley for one or two nights to visit some vineyards. However, we don't drive, so would need to use public transport or join an organised tour. We'd rather be independent if possible, so could anyone suggest a great location to stay in, and tell me how we would get there? We have a budget of $200 per night, plus the cost of any excursions etc. Many thanks!