A good day (in this order because it is the easiest):
Start at Sterling before it gets too crowded. The experience and scenery is great, but the wine isn't all that great.
After Sterling, stop in Twomey. It's small, but friendly, has great wine, and your tasting fee includes a branded wine glass as a souvenir.
Stop by Frank Family and enjoy their wonderful chardonnay and sparkling.
Head over to Oakville for some groceries and wine and picnic outside.
After lunch you can make your way to Chauteau Potelle (see my link above). There are a few other wineries, but if you work your way south you can hit Hess family last (it's open later than a lot of others).