I'm also in California, and just finished building my handful of chicks a coop as well. There are a lot of possibilities, right now I am using pine shavings for the coop floor. I have a bit left over from my brooder, and I've been taking them out to the run during the day for the last week. At bedtime, they would go back into the bucket, with the pine shavings. My thought was that they know pine shavings are found where they sleep, so until they are used to their new home I will try to lure them there with it.
Once I run out of that, I'll probably go with straw. That said, these pages are full of suggestions!
It's been super windy where I am, the chicks like to hunker down together when it gets blowing really hard.
What part of Cali are you in?
Edit: looked at your profile, we are neighbors! Oak Hills reporting in!
Thank you all! Would I need to have a light on for them at night as well, or are they okay without one?