All tracks:
At the Press Start screen, press R2, L2, Circle, Circle, Square, Square.

All cars:
At the Press Start screen, press R2, L2, Square, Square, Circle, Circle.

Kurt's Spy Car:
At the Press Start screen, press Square, Square, Triangle, Circle, X.

Smash Mouth car:
At the Press Start screen, press Square, Square, Square, Triangle, Circle, X.

Unlock Bonus vehicles:
Enter SMASHMOUTH as a code.