If you're looking to get away for a few hours and you love hiking and nature, you may want to consider visiting one of the Channel Islands (Anacapa, Santa Rosa, Santa Cruz, San Miguel and Santa Barbara...also known as the Channel Islands National Park).
The best way of getting to the islands is by boat. I've personally only visited Anacapa, it is the closest one to the "mainland" about 1 hour away by boat. There is nowhere to purchase food on the island, so I recommend you pack a lunch, drinks and snacks for the trip. Once you arrive on the island, it is yours to explore. If you don't mind the cold water, you can also snorkle or kayak, but you must bring your own equipment. Remember, you must enjoy nature and hiking if you plan on visiting Anacapa, otherwise, you might end up bored out of your mind.
Island Packers offers boat rides to the various islands at reasonable prices (e.g. a day trip to Anacapa will run you about $45 for adults and $28 for children). For more information, visit: http://www.islandpackers.com/isltrips.html
For more information on the Channel Islands National Park, visit: http://http://www.nps/.gov/chis