We rode the tour of Napa last year and will ride it again this year http://www.eaglecyclingclub.org/tour.htm
. There are route maps on this web site. In general, the elevation gained in Napa / Sonoma is 50 feet mile in the valleys and 100 feet per mile or more climbing the ridges. However, if you are not used to some climbing, then Napa can be a tough day in the saddle. Highway 29, which is the major road where the brand name wineries are located, is not great for cycling due to the traffic and rough roads. However, most of the roads are okay and some are excellent.
A month ago, we rode our tandem along Silverado Trail starting at Silverado Country Club and did 40 miles. This section of the valley is flat, 50 feet per mile, and has great bike lanes. You can ride along the Silverado Trail and visit a number of small wineries over a 40 to 50 mile ride. Last September, we did a 4 day tour of Sonoma which included a ride over the ridge via Highway 128 to Napa. Highway 128 is two lanes with a minimal shoulder and can have some high speed traffic. However, it was not a problem. The climb is of moderate difficulty and the descents medium in technical difficulty. I would include a ride from Napa to Sonoma to visit the Ferrari Carano winery http://www.ferrari-carano.com/
. FC has award winning gardens, architecture and wine – best in Napa / Sonoma. This could be ~75 miles or farther depending on where you stay in Napa.
If your level of conditioning and climbing ability is reasonable, then my suggestion is to bring the bike. You will love the riding. If you do not have a place to stay picked out yet, check out Silverado Country Club. They have hotel services and a great golf course and spa and it is located a couple of miles from the Silverado Trail for great riding and smaller but excellent wineries.
Whatever you can do, or dream you can, begin it. Boldness has genius, power and magic in it. - Johann Wolfgang von Goethe, writer