Angel's Island is this kickass, sort of deserted island with a mysterious past. Its in the middle of the SF bay, you have to take a ferry out to it, and they only have 9 primitive campsites (well, 10 if you count the kayak only campsite).
After the day tourists leave, you're basically the only one on the island and its really rad and spooky with all the abandoned army / immigration buildings that are in various states of decay.
Plus, theres an old missile silo on the island. So. Rad.