Pacific Coast Air Museum, Santa Rosa
Santa Rosa is home to this small air museum right next to the Charles M. Schulz Sonoma County/Santa Rosa Airport. It is small but it has an interesting selection of mostly old US military airplanes and helicopters, which visitors can go aboard, a nice treat for enthusiasts.
In addition, every year it puts on the "Wings Over Wine Country" airshow on a weekend in mid-August. This brings in operating aircraft, much WWII vintage, which over the years have included a B-17, a B-25, P-51 Mustangs, a Spitfire (if I remember correctly), and many others.