I live out in LA and the fishing is slim. If you have the time where you can travel up HWY 39 and fish the East or West Forks of the San Gabriel River. This is small stream fishing and close to the urban city so you can expect to see graffiti close to the road. On the West Fork of the San Gabriel you can fish the C&R section which is a small tail water so flows and water temps stay about the same year round. Your not going to be catching trophy sized fish but the rainbows are beautiful in colors.
Another river you can try is the Kern River about 2.5 hours north of LA in the southern sierras. Fishing can be quite good on this river if you get on the wild trout section above Johnsondale Bridge. Rainbows and browns in the 16-20 inch range.
Also the San Bernardino mountains has a few creeks wild small wild trout.
Deep Creek, Bear Creek and Santa Ana River.
For the small creeks a 00-2wt rod would be perfect.
There are lots of dams up here as well. If you can rent a small boat you could get into tons of bass, strippers, sunfish and trout.
Also lots of guys are taking up fly fishing in the surf. That type of fly fishing is a whole new ball game out here. Corbina, halibut and leopard sharks seem to be some of the more popular fish to target. Oh and surf perch as well.
See it's not just a visit to the World's largest fly fishing store Bob Marriott's. You could actually C&R a few fish while in LA. I hope this helps you out on your trip out here.