On a cool morning, my girlfriend and I took a quiet and solitary hike through the Bale Grist Mill State Historic Park near Calistoga and Saint Helena. The park itself is beautiful and has a couple of very nice hikes, we chose an easy one because we wanted to get out and see the wineries for much of the day. But I will say, the mill itself was pretty cool. Although it was closed, we were able to walk around it and see much of the historic aspects.
Fair warning, you'll have to pay a park official to park for the hike.