A few ubiquitous California plants:
Platanus racemosa, or California sycamore
I think the bark on this sycamore beats our London plane, hands down. And the fall color is certainly better.
Pinus pinea, or Italian Stone Pine
Callistemon citrinus, or Bottlebrush
Clivia grew all over southern California,
...as did the ubiquitous and lovely Bird-of-paradise, or Strelitzia reginae.
This was my first trip to LA. I'd been to San Diego & San Francisco before, but never Los Angeles. The massive area that the city takes up is startling. Rationally, I knew it was big (500 square miles, as opposed to New York City's 300), but I still couldn't understand the scale of the place too easily.
I was really surprised, too, to never lay eyes on the Hollywood sign. When I got there, I thought it would be omnipresent -- like the Eiffel Tower or the Empire State Building, or Big Ben, even. But no. Granted, though I was only in LA proper for about 12 hours, I never encountered it.
I did, however, get to see Randy's Donuts, which was actually pretty exciting.
I wrote my thesis for grad school on roadside vernacular architecture, and even mentioned Randy's Donuts, in addition to Lucy the Elephant and the Long Island Duck. It's the whole building-as-sign thing that grew in popularity in tandem with the automobile industry. I didn't plan on making it a destination during my short trip (and didn't even sample the donuts), but I think I was more excited to see this than had I seen the Hollywood sign...