My last little bits will hopefully help to make your trip a little easier, a little more enjoyable and very memorable.
Visit mid-week. There's a lot less traffic and some hotels have lower rates which means more money for food, wine and fun activities.
Speaking of food, wine and fun activities...my idea of a perfect fit would be dining at French Laundry or one of the other fine restaurants in the area. If you don't have a reservation and decide on a whim (sounds familiar), head over and prepare to wait - most likely, they'll fit you in. But don't quote me on that, always have a back-up just in case! :)
Try using Silverado Trail as your north-south driving route. It's more scenic and less crowded.
Rutherford Hill Winery's picnic area has a great view, and is nearly empty when picnic areas on the main highway are packed. My ideal picnic? ***Baguette, at least 3 cheeses, fresh fruit and of course, a great bottle of wine*** As a courtesy to the winery, buy a bottle of wine from them to enjoy with your picnic (they'll open it for you if you don't have an opener).
By the way, a great place for picnic goods is Sunshine Market on Main Street in St. Helena (pronounced Hel-EE-na) which is about 30 minutes north of Napa. You'll find an excellent selection of cheeses, fruits and baked goods. Don't forget to grab napkins and utensils while you're there.
While in St. Helena, check out Steve's Hardware for fun kitchen tools and gadgets ranging from tiny tart pans to the biggest wooden spoons you've seen in real life - I'm talking Harry Potter movie style.
PS - If you're near Yountville in the morning (about half way between Napa and St. Helena), stop by Gordon's for coffee with the locals...it's a great way to learn more fun secrets of the area.