If you are attending street fairs, public festivals, and other city events in California and want to use a hidden camera is that legal to use?

Using a camera surreptitiously could raise a few issues. First, in many states, it is illegal to record someone without their permission. There are exceptions and these vary from state to state. The next issue concerns what you plan on doing with the footage. If you plan on using it commercially, then the people shown in the video have a right of publicity that prevents you from using their images/voices for commercial purposes without their permission. Again, these rights vary from state to state so you need to find out under the laws of the state(s) you intent to use the footage whether the subjects of your video have enforceable rights.