On the main start menu, enter the following:
Unlock All Drag Tracks - Right, Square, Left, R1, Square, L1, L2, R2 
Unlock All Drift Tracks - Left, Left, Left, Left, Right, R2, R1, L2 
Unlock All Circuit Tracks - Down, R1, R1, R1, R2, R2, R2, Square 
Unlock All Sprint Tracks - Up, R2, R2, R2, R1, Down, Down, Down 
Unlimited Nitro - X, CIRCLE, TRIANGLE, TRIANGLE, X, X, CIRCLE