Press 'P' to pause the game, now press and hold down 'G', "U", and "N". The screen should flash indicating infinite everything, now pressing 'F1' through 'F5' will jump to the corresponding level and 'F6' will show you the end sequence.
"442244" - Paradise Island
"867377" - Fairground
"835000" - Sky Fortress
"730098" - Space Mountain