Tom Sienstra's book would be an excellent resource for you:

http://www.tomstienstra.com/books-html/cal-camp.html

I don't have my copy on me right now, but he lists many walk-in campsites. These are usually campsites that are a short distance from the parking lot (1/4 - 1 mile). It is usually over flat ground so I think it would be perfect for you needs.