I went up to the North Bay wine country last Thanksgiving and it was really beautiful. The leaves were still on the vines but they had changed colour. It's our California version of the East Coast Fall colours.

So the wineries are a nice tour. Just make sure that you don't go on the Sunday of Thanksgiving weekend. The traffic coming back to the city will be horrible.

You can also visit wineries in the South Bay (although in my opinion you don't get the full 'autumn leaves' effect in the south bay). The South Bay is the home of Ridge winery....which just trounced the French in the 'judgement of Paris, 30 years on' tasting http://en.wikipedia.org/wiki/Ridge_Vineyards

Don't know if you like wine, but if you do, it's a good time to visit.