A million things come to mind concerning a Napa valley tour and lots of it has to do with, first, where you are staying, and second, how much wine you want to consume. While I freely admit to overdoing it when I visit (my brother lives there) I definitely have some favorite wineries. However, since you are visiting, I need to know what your favorite wine types and wineries are.
One of the finest restaurants is in Yountville and called The French Laundry.. and you need early reservations. It's expensive but perhaps you owe yourself one really elegant dinner while you are there. It is very popular.
The worst thing you can do is try and cram too much into a short time. Rather I suggest you choose carefully and enjoy the time at each, allowing lots of things like picnic time and just enjoying being in the area. It truly is a delight.

Eleuthera is right the best thing to do is figure out what you want to taste! Whether it be sparkling wine, pinots, old vine zinfandels, etc. There are a lot of vineyards and tasting rooms. If you can't get into French Laundry or don't want to spend the money, give Bouchon Bakery. It is run by the same guy and have some of the most heavenly bake goods I have ever tasted! The homemade nutter butter is out of the world! Hope that helps and can't wait to hear all about your trip!