Up a beautiful, but nauseatingly winding road from Calistoga is Mount St. Helena — enter through the Robert Louis Stevenson State Park (where Stevenson lived in a cabin and wrote Silverado Squatters in 1883) and trek five miles to the top. The first section involves a bit of scrambling, but most of it is along a fire road.
The view of the Napa Valley — and, on a clear day, the San Francisco Bay — from 4,000 feet makes it well worth the effort. Get an early start and you will make it to the top in time to see the fog begin to lift from the valley floor. We highly recommend bringing a bottle of Schramsberg and a breakfast sandwich (Gott's Roadside in St. Helena opens at 7). Afterwards, stop for a well-earned beer at the Calistoga Inn.