A great beach close place to stay is Foster Park. It is a very small Ventura county campground with 10 full hook up sites and 6 tent sites. The FHU sites are $36, a real bargain in SoCal. It is about 10 easy miles up highway 33 from Ventura beach tucked away in a canyon just below lake Casitas. A short drive to anywhere in Ojai, Oxnard, Ventura and not too far from Santa Barbara. The good thing is not too many people know about it (hope I am not ruining that!) and they take reservations online. They still have plenty of June and July availability (I just looked) especially midweek. Sites 8,10, and 2 are the best IMO. This is our weekend getaway place, about an hour from home. Silver Strand beach in Ventura is very nice and liked by surfers. Let me know if you decide to head to Ventura, I work out here and know some people who surf.
If you are looking for more L.A. centric, I agree with Valencia travel village. Magic Mountain Hurricane Harbor, Reagan Library, Natural History Museum, California Science Center, Ventura county beaches, Griffith Park Observatory, Travel Town, etc. all relatively close by.