The Sonoma County Bicycle Coalition produces a wonderful map of recommended (and not) streets and trails for riding, but here are some of our recommendations:
West County Trail:
The West County Trail runs between Sebastopol and Forestville. This ~5.5 mile section of trail starts near the intersection of Mill Station Road and Highway 116 in Sebastopol, and ends in Forestville. Access to the trail is located off the following public streets: Highway 116, Occidental Road, Grey Street, Graton Road, Green Valley Road, Ross Station Road, Ross Branch Road, and Pajaro Lane.
Joe Rodota Trail:
The Joe Rodota Trail runs between Santa Rosa and Sebastopol. The trail starts at the pedestrian/bicycle bridge intersection with the Prince Memorial Greenway (between West 3rd Street and Railroad Street), southwest of the Marriott Courtyard Hotel. The trail ends near the intersection of Mill Station Road and Highway 116 in Sebastopol. Access to the trail is located off the following public streets: Roberts Avenue, Dutton Avenue, Stony Point Road, Wright Road, Sebastopol Road, Merced Avenue, Llano Road, Petaluma Avenue, High School Road, Cleveland Avenue, Ellis Court, DuFranc Avenue, Hurlbut Avenue, and Highway 116.
Armstrong Redwoods State Reserve in Guerneville:
Park in the free lot just outside the park gates and ride your bike on the paved roads and trails (bikes are not allowed on unpaved trails).
Challenging Rides:
Two challenging rides in the county are the Kings Ridge-Tin Barn Road ride near Cazadero and Coleman Valley Road near Occidental. The Kings Ridge ride, favored by pro racer Levi Leipheimer, is a challenging, hilly course. It is considered one of the most beautiful, fulfilling bike rides in the world. The Coleman Valley ride is a landmark climb in Northern California and has been featured in previous routes of the Amgen Tour of California.