Santa Rosa Plateau
Not many people know about this place, run by the Nature Conservancy. It has one of the best remaining examples of oak-savanna habitat, with native Stipa grasses.
It's easier to hike in than the mountains, with only low rolling hills, and it's beautiful after the winter rains, with the California poppies and other wildflowers blooming.
The reserve also has vernal pools -- little pools that stay wet all spring and summer and have their own unique ecology.
It's at 22115 Tenaja Road, in Murrieta, Riverside County. Finding it is tricky, so use a map.