X, R-TRIGGER, Y, R-TRIGGER, L-TRIGGER, R-TRIGGER, R-TRIGGER, R-TRIGGER.

You will unlock every game mode.

Unlock Secret Cars
At the Press Start screen enter.

X, Y, R-TRIGGER, R-TRIGGER, R-TRIGGER, R-TRIGGER, X, Y.

You will unlock every Secret Car.

Unlock All Extras
At the Press Start screen enter.

L-TRIGGER, R-TRIGGER, X, L-TRIGGER, X, R-TRIGGER, X, Y.

You will now have access to all the Extras.