This is a hike to the highest peak (4339 ft) in Sonoma County, though it’s only about 1500 ft or so away from the Napa County line. This trail follows a gently graded fire road, so in cool weather it’s fairly easy despite the distance. The hike is 10.2 miles roundtrip, with an elevation gain for around 2300 ft. Parking is free on the side of the road.
For more details and directions, check out the Norcalhiker blog post for this trip.