The Napa Valley offers a wide variety of bicycling options.
The Innkeeper and his wife, Lorinda, have done them all: the flats, mountain loops, off road and single tracking. The flat valley floor is perfect for either touring the wineries or for a long relatively flat ride. Hwy 29 is awfully busy and quite narrow in spots, therefore the Silverado Trail is really the preferred route. Since the valley is over 30 miles long, you can get some really good long rides in.
As soon as you get off the valley floor, you begin climbing. The backroads to Angwin make a great mountain ride; it’s only 10 miles round trip, but you climb 2,500 feet and then get a great coasting ride home. For a great long road ride, you can extend the Angwin ride down to the next valley of Pope Valley, wind through Chiles Valley and back to the Napa Valley. This is 35 miles of backroads with one great 4 mile stretch of down hill banked turns that is an absolute gas!
For off road options there are three: Bothe State Park with 3 or 4 trails (one of them goes 7 miles back into the woods), Mount St. Helena which has one crisscrossing fire road that climbs to the top of the mountain, and a quiet spot up in Angwin that has 25 miles of fire roads and single track.
Check out St. Helena Cyclery for a great local bike shop with rentals. Visit these sites for tours: Getaway Adventures and Napa Valley Bike Tours.