US A333 PIT,
You will most likely be in Terminal 4 at LAX (I think they only use Terminal 3 for short haul west coast stuff). If you head to the south end of the terminal you will be close to runways 7/25 and you can watch the action from there. You won't be able to see anything on the north side (runways 6/24) though.
You could try heading up to the observation deck on top of the theme building if it's open, but then you'd have to go back through security, which is more trouble than its worth.
Also note the existence of the underground tunnel airside at Terminal 4, it goes to Terminal 5 (mostly Delta) and 6 (some Delta, some United, Continental, National) if you want to check out those terminals. In my opinion Delta's Terminal 5 is the nicest at LAX, and Ruby's in Terminal 6 has good food. Once in Terminal 6 you can also get to Terminals 7 and 8 (both United), all without having to go out past security.
David / SAN