I agree with W. Dry, it is a great easy tandem ride. We did it as a 40 milers last year. Started in Healdsburg, went north on W. Dry, up to Lake Sonoma (that's a long climb so I would not advise it), south on Dry for a little while and east on Canyon (moderate climb) to reach Geyserville. We then took 128 through the Alexander Valley and came back to town via A.V. Road.
Alexander Valley road was a bit busy so not the best ending, the rest was splendid.
I also agree with Silverado. Sure it is flat but the traffic makes it very unpleasant.

I am thinking about the Anderson Valley as a potential ride. Does anybody has experience riding it?