Mount Diablo State Park near Danville.
"This park is one of the ecological treasures of the San Francisco Bay Area. Every season in the park has its special qualities. Discover for yourself the mountain's beautiful wildflowers, its extensive trail system, fascinating wildlife and distinctive rock formations. View the stars from its lofty heights, bike ride to its 3,849 foot summit or explore the more remote trails by horseback. The park offers hiking, biking, horseback riding and camping. " http://www.parks.ca.gov/?page_id=517