I'm often asked about day trips to Napa. Here's my preferred winery tour:

First, to set the scene, check out my guide to wineries map on wayfaring.com. It contains map locations, addresses, and detailed descriptions of my experiences at various wineries. Here's the link:
On to details:
Start out for Napa around 10am to give yourself enough time and avoid some of the crowds (the route is designed to keep you out of traffic).
Drive to Napa, be sure to bypass the city of Napa itself since there's nothing of interest there. Shortly after the expressway ends, you'll come to Oak Knoll Road. Turn right there and on your left is your first stop Trefethen Winery. Make sure you get a library tasting there (more expensive but worth it). At that tasting, you'll get to sit on big leather couches in the private back room of the winery and have some fantastic chardonnay and cabernet. Don't be shy to ask to taste any wine you'd consider buying.
Continue up Oak Knoll Road until you come to the Silverado Trail. Turn left and drive to Rutherford. On your right is Rutherford Hill Winery which is located atop a big hill overlooking the Valley. Stop in for a tasting and a tour of the wine caves here. Choose a bottle that you liked, buy it, and head outside to the olive grove for a picnic lunch at a table with a view of the entire Napa Valley. Ideally, you'll have brought some cheese, baguettes, or other foods that pair well with wines. You might also bring your own wine stems (Rutherford Hill will supply plastic cups for picnics but not upscale glasses.)
Next, go back down the hill and turn right onto Silverado Trail. Drive up to Lodi Lane and turn left and then immediately right to visit The Duckhorn Estate. The Duckhorn family graduated from Haas, so if you meet any of them (and it's possible you might) be sure to tell them that you're a recent Haas grad. Here you will have a seated tasting in an airy room with windows looking out over the winery. On the wall is a large mural of ducks flying over a pond. Be sure to get the more deluxe tasting here so that you can compare the wines produced from grapes grown all over the valley to wines produced from a single vineyard. You should also ask to try the Goldeneyethe best Pinot in Napa in my opinion. 
Turn right out of Duckhorn and head down Lodi Lane until you come to highway 29. Turn left and head toward Napa. On your right is the Mondavi Winery. Turn in here for a quick stop. The Mondavi grounds are spectacular especially the rose garden. Be sure to take some pictures here in the afternoon sun. After around 3pm, the sun is sufficiently low in the sky that the whole place is bathed in gold. If you missed the tour at Rutherford Hill, you have a chance at a tour at Mondavi as well. Mondavi doesn't really do proper tastings instead, grab a glass of merlot, sit by the garden and relax in the afternoon sun.
Turn right out of Mondavi onto highway 29 to head toward Napa. Along the way, you'll come to Oakville and, on your right, is your last stop Domaine Chandon. Here, you'll be tasting sparkling wines on a patio overlooking the beautiful gardens of the Domaine. Be sure to do a sample tasting so you can compare a brut to a blanc de blanc to a blanc de noir style of sparkling wine. (Hint: they differ in their blend of chardonnay and pinot noir, the two grapes used in sparkling wine.)
You might be in the mood for dinner and the Domaine has a lovely restaurant. Make a reservation here: http://www.opentable.com/single.aspx?rid=2410&restref=2410 One thing that's nice about the restaurant is that there's no markup for wines, so, while food is expensive (but worth it) you won't get hit with a big wine bill on top. 
After dinner, it's time to head home. Fortunately, the traffic will have all cleared out while you were having dinner, so it's smooth sailing on your way back.