Columbia California has a nice trail from the airport to the historic gold rush town and state park of Columbia. It can be a hot in mid-summer, but it is very enjoyable walk at other times. We went there with our kids and my dad, and had a great time.
Update: During the recession many of the businesses have closed in town. Its still a great place to hike and spend some time, but there are reports that it is a little less lively than usual.