Planet Todd:
Press 4 +2 at the world selection screen.

Debug mode:
Press 8 + 6 during game play. Three rows of hexadecimal debug information will be displayed. The "Drop Camera" and "Fixed Camera" views may now be selected in all game modes.

Atari headquarters:
Begin game play on the Wild-West World in single player mode. Drive into the pulsating wall inside the mine shaft.

Castle:
Enable the fast car option and begin game play on Jerome's World in single player mode. Drive into the first floor fireplace.