Napa is known for its wine, which means there are many places to tour and taste. What would you say are your favorite places for tastings? Do they tend to be free or do you need to pay in advance? I've been to both kind of wineries, and it's nice to know in advance before getting to Napa.
I suggest getting a priority wine pass as it will allow you free tastings as well as special VIP treatment. The yearly pass costs $125 but you can get it for $40 using the promo code "localwally". The pass is good for both Napa and Sonoma and pays for itself within the first hour so I recommend you get the card and go have fun.
I remember the days before Napa Valley became what it is today when tasting and winery tours in Napa were free. These days wineries charge up to $30 per person to taste their wines and around $70 to tour their facilities. My tip is to visit outside of the high-season to benefit from cheaper tours, airfare, tastings, etc.