Oh what's rain gonna do to you?
Bring an umbrella and you'll be fine!!
You can still visit wineries and taste their wines in their wine tasting rooms. Some wineries' tours are completely inside, with access to their caves from their tasting rooms. Google a few.
There is a great outlet mall in Napa (city)
http://www.premiumoutlets.com/outlets/outlet.asp?id=25. St. Helena has the cutest downtown with shops to peruse. Foot Candy is my favorite!
http://www.footcandyshoes.com/istar.asp?a=30
Plus Napa has a huge number of fabulous restaurants. Try
Ad Hoc in Yountville (it's a Thomas Keller restaurant), or
Mustard's Grill in Napa or the bar at
Auberge du Soleil in Rutherford off the Silverado Trail or the
Rutherford Grill in Rutherford off 29 or
Cindy's Backstreet Kitchen in St. Helena for more casual places.
The Martini House or
Greystone (CIA) or
Meadowood all in St. Helena are great for a fancier setting.
French Laundry in Yountville is one of the best rated restaurants in the US but you usually have to make reservations 3 months in advance.
Since it is a holiday weekend, make sure you call before or use Opentable.com to make reservations.
Have fun!