It's all quite beautiful. I'd recommend Nevada County (in the northern part with Grass Valley and Nevada City), Amador County (Sierra Foothills - GREAT up-and-coming wineries!) and Mariposa County (near Yosemite).
We did a couple day trips in the Sonora and Yosemite area. It was fun....but not anything that'd I would spend more than a day or two on. Did some gold panning and walked through some of the old towns. It was interesting though.