All cars
At the Press Start screen, press R2, L2, Square, Square, Circle, Circle.
All tracks
At the Press Start screen, press R2, L2, Circle, Circle, Square, Square.
Kurt's Spy Car
Enter Square(3), Triange, Circle, X.
Smash Mouth car
At the Press Start screen, press Square, Square, Square, Triangle, Circle, X.
All cars...