I am not a lawyer, but I do live in California and I have been a director of engineering when this question came up.
You are in California so non-compete wouldn't apply even if you signed it. We also have off-duty conduct laws here, so any lawful act you engage in outside of work is not a fireable offense. So if you want to drive for Uber or do some contract app development, you can do it if you want. I would not mention it to HR or legal, however. While they might not be able to legally fire you specifically because you are contracting on the side, you are an at-will employee and they can use the fact that you asked about it as a reason to fire you (the same as telling your boss "I'm interviwing"). They might discuss it amongst themselves and quietly terminate you without stating a reason.