My favorite riding in that area is around Healdsburg. Go to the bike shop there (Spoke Folk) and say hi. They have rentals (including tandems) and will give you a little map with a bunch of local rides leaving right from the shop. My favorite is West Dry Creek. Very mellow and lots of cool wineries.
Also check out the Santa Rosa Cycling Club website for a list of the ten best rides in that area.
Someone mentioned Silverado- from there, I would recommend the Chile's and Pope's Valley loop. There is a route sheet on the Oakland Yellowjackets website with a bunch of different options, depending on the distance and climbing that suit you.
This is some of the best riding in the state. You will not be disappointed. For longer routes, check out the Wine Country Century route. You could easily follow that on your own.
Good luck and have a great trip!