I prefer Sonoma to Napa, but you can easily do both. Someone here turned me on to David Coffaro in Dry Creek, which I'd recommend. He's a madman Raiders fan who wears sleevless shirts and grows shite most in Dry Creek wouldn't, but he makes some really good-- and very affordable-- wines. Ask the girl in the tasting room to give you some barrel tastings and get info on his futures, as they're an amazing deal.
In Sonoma, off the top of my head, I'd also recommend Unti, Rafanelli, Copain (the guy who runs the tasting room is a NOLA transplant), Rochioli, and Emeritus. In Napa, I'd definitely make a tasting reservation at Phelps to enjoy the wine and fantastic view.
As other posters said, there are many craft brewers in the area as well, so if you're a beer geek, don't miss your chance to at least stop by Russian River Brewing.
Oh, and eat at the Fatted Calf in Napa for lunch one day, and slip into a heavenly meat-induced coma.