I lived there for 30 years and I agree that the C&R section of the West Fork San Gabriel is your best bet for now. I used to regularly go to the upper East Fork years ago and it can be surprising at certain times of the year if you hike in past the end of the road.
A good all around attractor dry fly there is a light cahill. I prefered tied parachute. A gold ribbed hares ear for a nymph is also a good one (isn't it always?).
You'll likely run into poachers, but considering the types of people resident there now, don't confront, just report.
I left there for the Central Coast in '86 but returned several times untill a few years back when we moved to TX. Yes, don't leave anything valuable in your car and if it's an older one, so much the better. Also, you might get disgusted at the trash and crowds in the first mile of stream, don't lose heart, it gets better.
There's also Deep Creek in the San Bernadino Mountains. It's a nice C&R creek that can be kind of hard to find. Don't ask me for directions though, I only went there twice and my memory is bad, ask the Forest service, they can get you there.
Have Fun
Charlie