I lived in the Napa Valley for 16 years. Napa Valley wine country has been mentioned, as were the redwoods (which are definitely worth driving north of San Francisco to see (lots of beautiful ocean coastline to see as well). If you're into that sort of thing, I'd suggest the Monterey Bay Aquarium as well---well worth checking out.
Yosemite is a very good idea. As I mentioned to Trep a while back, we have had a lot more rain/snow here this past winter than we have had for some time--and the waterfalls in the park are more spectacular in Yosemite right now than they have been in years. You couldn't have picked a better year to see Yosemite than this one.
I'd also recommend Lake Tahoe as well. Beautiful, with something for everyone. Plenty of scenery, and lots of nightlife and casinos if that suits your mood.
Hope you have a good trip.
