I finally completed all 9 California National Parks with an overnight camping trip (and a broken foot!) to Santa Cruz Island. Channel Islands NP was definitely a hidden gem in my opinion. Santa Cruz Island is the largest and most-developed island of the 5 islands of this NP, offering breathtaking coastal and valley views, unique wild lives, and interesting sea-cave kayaking experience.
As an additional bonus: we had curious dolphins joining us riding the waves side by side with our ferry!