Unlock all game modes
Go to the 'Press Start' screen. Press X, R trigger, Y, R trigger, L trigger, R trigger, R trigger, R trigger to be granted every game mode.
Unlock secret cars
Go to the 'Press Start' screen. Press X, Y, R trigger, R trigger, R trigger, R trigger, X, Y to be granted all secret cars.
Unlock extras
Go to the 'Press Start' screen. Press L trigger, R trigger, X, L trigger, X, R trigger, X, Y to be granted all extras.