Janos, I used to live in Healdsburg and drove up to the Geysers everyday. Alexander Valley (East of 101) is also flat and pretty with a fair amount of wineries. Ride north on 128 from Chalk Hill all the way to Cloverdale. Grab lunch at Jimtown or Geyserville.

I didn't ride when I lived there, but I would not suggest riding 128 south of Chalk Hill. As I recall, it was narrow