If a Bay Area day hike with some amazing views is on your list of things to do, Castle Rock State Park should definitely be a consideration. Rock climbers, hikers, and backcountry campers can all find enjoyment here. With over 6 miles of hiking trails, multiple climbing access points, camping, and vistas all the way to the Monterey Bay, you can be sure you can fill your day with things to do.
Two main trails connect to make a loop that circumnavigate the park. Saratoga Gap Trail and Ridge Trail are your main thoroughfares with shorter, interconnecting trails between the two. A shorter loop taking you to Castle Rock, the geological formation from which this park gets it's name, can be found from the Saratoga Gap Trailhead via Castle Rock Trail or from the parking lot.
Saratoga Gap Trail starts off an easily meandering through trees and around boulders as it proceeds toward the Castle Rock Campground. About 1.5 miles down Saratoga Gap you will intersect with the Ridge Trail Trailhead. Here you can choose to take this to Goat Rock, Interpretive Shelter, or just continue down Saratoga Gap to the campground. Either way you will eventually end up at the Castle Rock Campground where you can rest and fill your water bottle.
Saratoga Gap Trail and Ridge Trail both have amazing views of the Santa Cruz Mountains that reach all the way to Monterey Bay. One moment you are under the cover of the tree canopy, and the next you break out into the open with the trail hugging the ridge. The trails are well marked and accurate, so the trail is not too difficult to navigate. However, there are a few places where you'll need to scramble over boulders. There is even a section where stairs have been cut into the hillside and a wire handrail has been attached.
Make sure you fill up your water bottle as you prepare to leave, as there is no place to fill it from the parking lot. You must hike to the campground if you wish to have access to water. Also, depending on the day, the sound of gunfire from the nearby gun range can be tough to ignore. Nevertheless, between the trails, geology, and climbing opportunities, this is a very worthwhile park to explore.