This is my personal favorite, a 60 mile gem. http://ridewithgps.com/routes/9157
I drive an hour to ride it 2 or 3 times a year, I like it that much.

It starts at a Starbucks right off the freeway for convenience.
2007 Redwood Rd
Napa, CA 94558

Then there is a nice little flat warm up before climbing Mt. Veeder, which is pretty and has very little traffic. The Oakville Grocery in Oakville is a great stop both out and back for bathrooms water and incredible deli munchies. After the store Silverado Trail is nice and the loop around Lake Henessey is pretty too. Bathrooms at Lake Henessey, no water.
After a climb up 128/Sage Canyon be sure to look for the Nichelini Winery on the right side of the road. Really a jewel, family owned and oldest winery in Napa Valley. Nice place to rest with water and bathrooms. Then the Chiles Valley/Pope Valley loop is real pretty too.

You will take in the whole valley on this ride plus get a taste of the hills on both the East and West sides of the valley. Just be ready for some pain and suffering coming back up the Oakville Grade on the way home as you go towards Dry Creek Rd.
The Oakville Grade is one mile of pure nasty. Dry Creek Rd is a nice payoff though, almost all down hill back to the Starbucks and your car.

Napa Valley as seen from the top of Oakville Grade