Infinite turbos 
At the main menu, press 
R2, L1, Square, Triangle, R1, L2, L1, R2. 
If you entered the code correctly, you will hear a sound.

TowJam car 
At the main menu, press 
Square, Triangle, L1, R1, L2, R2, Square, Triangle. 
If you entered the code correctly, you will hear a sound.

Small cars 
At the main menu, press 
Square, R2, L2, Triangle, Triangle, L2, R2, Square. 
If you entered the code correctly, you will hear a sound.

Large tires 
At the main menu, press 
Square, Triangle, Square, Triangle, R1, R1, L2, L2. 
If you entered the code correctly, you will hear a sound.

Flat shaded graphics 
At the main menu, press 
L1, R1, L2, R2, L1, R1, L2, R2. 
If you entered the code correctly, you will hear a sound.

Dude sounds 
At the main menu, press 
R2, R1, L2, R2, Square, Triangle, L1, R1. 
If you entered the code correctly, you will hear a sound.