There are a couple of outfits around who will rent you aircraft in California. If you go to
www.beapilot.com there is a list of flight schools that will almost certainly help you. Also you could try an aircraft sales company that may well rent you an old 172 that they're trying to shift?
Before you go it may well be worth chatting to an instructor about some challenging flying to be done. Getting into Santa Monica airport crossing the LA basin or San Francisco? There's plenty of space over there and its possible to fly 100hrs and never enter a controlled zone if you didn't want to. Radar advisory is free and you may as well play with it!