One of your first stops should be Downtown Napa Association. Here, you'll be able to orient yourself and pick up some great ideas before you set out exploring.
This tourist information center is in Downtown Napa, a place that has plenty of things going for it! Artesa Winery, Napa Valley Wine Train, and Oxbow Public Market are other popular sightseeing opportunities close by when you've finished up at Downtown Napa Association.