The parking garage across from the Int'l Terminal at LAX is a good spot too. Through a staircase you can access the roof and you get good views of planes landing & taking off on both runways, as well as close ups of international carriers.
The airport hotel at MIA has a bar with good terminal views. There is/was an outdoor observation area but the last time I was there (a couple of years ago) it was closed, I think, for renovations.
The top of the garage at MCO is also a pretty good spot. An airport cop came up there & watched me for a few minutes before realizing my binoculors were harmless, got bored & left.
Not like you want to watch planes in Hawaii, but the oceanview rooms at the Sheraton Waikiki allow neat views of planes taking off over the ocean.
Can you tell I'm easily excited by this topic?