Mount St. Helena is the big peak (4343 feet) just North of Calistoga. There have been plans to develop a state park there for years, but not much has happened. At the top you can choose the South or North peak. Both peaks have various man made structures. The North peak is a little further and higher. On a clear day you can see Bodega Head (the ocean), SF, Mount Diablo, the Sierras, Lassen and Shasta. One of the best views in the world. The trailhead is at about 2000 feet. Take lots of water, there is none on the Mountain. It is a six mile ride to the top of the North Peak. An intermediate rider can make it to the top in under 2 hours. The trip down takes just 20 minutes. A fun variation on this ride is to do it the night of a full moon. The trail is well illuminated by the moon. However, it is still advisable to have a bike light with you for the trip down.
To get to the Hiking trailhead, follow Highway 29 East out of Calistoga and up a very windy road with many switchbacks. Right at the saddle are the parking lot and hiking trailheads. Keep going about 1/2 a mile and you will see where the fire road intersects with 29. There are several pull outs where you can park. Bikes are not allowed on the hiking trail from the main parking lot. The hiking trail joins up with the Fire Road about 1.5 miles into the ride.