cheap/free camping in socal?

I'm looking for a campsite that I can go to with some friends for a few nights this month. I haven't bothered looking at any state park sites because I know they get booked months in advance and this is a last minute trip we want to do during our spring break. we really want to find a beach area but are aware of how unrealistic that is. I have some camping experience so I'd be willing to go somewhere without amenities or somewhere we can car camp. any suggestions? thanks! :)