We recently visited Castle Rock state park, an old favorite that we hadn't been to in many years (since they don't allow pets). The park at the crest of the Santa Cruz mountains features several huge rock formations with many small caves and crevices to explore. Unfortunately the park has gotten so popular that we had to park a mile a way and hike in, but the kids enjoyed the hike and had great fun finding "bunk-beds" in the rock formations.