We did a similar trip for our honeymoon and here are some things we loved in Napa:
Candlelight Inn: AMAZING breakfasts (that will keep you going most of the day), plus a pool and this to-die-for chocolate cake that is available all the time from the kitchen. I dream of that chocolate cake and I'm not even a chocolate cake person.
www.candlelightinn.com
Coppola Winery: Not very expensive for the tour, which comes with a sampling and cheese tasting on the estate. The tour was really neat if you're into movies; you get to see lots of original props and the backstory to the vineyard is really interesting.
www.franciscoppolawinery.com/
Rutherford Grill: Not too expensive and sooo good. They make a skillet cornbread that you'll remember for the rest of your life.
www.yelp.com/biz/rutherford-grill-rutherford
We also rented bikes and just biked around to different places for tastings. Very fun and it's mostly flat so it's hardly any effort. The views are beautiful.
I'm so jealous :-)