Wine is the first order of business. Most wineries offer tasting for a modest fee. Some offer tours or other attractions on site. I would guess there are something like 300 wineries in the area, and I have only visited a handful. My favorites so far are Hess Collection (Napa, decent wine, small interesting modern art museum), Cuvaison (my favorite wine), Korbel Champagne (Russian River, nice flower garden), Buena Vista (Sonoma Valley, nice picnic area), Sterling Vineyards (Catistoga, tram to great view – wine not so good), Clos Pegase (Catistoga, some interesting art).
Check of napa valley events for seasonal events and activities. There are a number of popular activities in the area: bicycling, hot air balloon rides, mud baths in Catistoga, etc. There are a large number of excellent restaurants.
Auberge Du Soleil is an outstanding place to eat and stay and The El Dorado Hotel looked very nice. There are countless B&Bs in the area Many are mediocre, but there are some which are outstanding. Alas, I haven’t visited many of the truly great places to stay because you need to get reservations reasonably far in advance, and I head up to the Napa area more or less at the last minute.