Old Faithful Geyser of California is a small but reliable geyser in the volcanic country north of Calistoga, erupting at intervals of roughly twenty to thirty minutes. Each eruption sends a column of water and steam up to twenty metres into the air for about three minutes. It's not Yellowstone, but it's a pleasant short visit on a Napa Valley tour.
The site includes a small petting zoo with rare four-horned Jacob sheep, a couple of picnic areas, and a small geothermal exhibit. Entry is paid but inexpensive. Visit Old Faithful alongside the nearby Petrified Forest for a half-day's geological excursion.