Corrected entry: In the drive-in scene, just after Sandy has rejected Danny and slammed the car door into his privates, look very closely at the car door. At the end of the movie, we see the car is a bright red colour which is covered with lots of dust during the rest of the movie. In this series of shots you can see 'John's' written into the dust (using a finger, obviously) on the car door, obviously referring to John Travolta. This is best seen in the digitally remastered version.
Corrected entry: In the last song there is a shot over the fairground which shows a spinning rotor cage full of people. Back on the ground two seconds later the rotor cage is clearly visible in the background and obviously empty.