Have you settled on Napa or Sonoma? As a starting point, the best places for Chard are Napa or the Russian River Valley in Sonoma.
Napa and Sonoma are too far apart to do both in one day, but if you have a couple days (and your friends have the endurance) you can do Sonoma one day and Napa the next.
Also, what are you price points? And do you want to be able to just drop in or are you cool with making appointments?