The metal detector scene always gets me. The same scene in Airplane II was nice to watch also. The "Smoking" ticket is one that I don't know if current generations even understand.
Also the one where the guy says "Striker, I'd watch my step if I were you!", then he goes through the door and you here him fall and lots of crashing noises.
The entire movie is full of gags. If you get a chance, there was a version of it I watched with pop up messages with commentary on all the jokes and sight gags included in the movie. There was something in every scene.