Press Square, Up, Down(2), Square, Circle(2), Triangle, X during game play. A beep will confirm correct code entry. Once debug mode is active, enter one of the following codes during game play to activate the corresponding feature:

Win current race
Press Select + X
Change view
Press Select + D-pad
Zoom in or out
Press Select + L2 or Select + R2
CPU control of player's car
Press Select + Square
Destroy all cars
Press X + Triangle + Circle + Square

Additional races:
After completion of the "Beginner", "Tricky", and "Difficult" groups, an "Advanced" group of races will become available.