Enter the following at the Main Menu. You'll hear a sound to confirm.

Infinite turbos     R2, L1, Square, Triangle, R1, L2, L1, R2

Big tires           Square, Triangle, Square, Triangle, R1, R1, L2, L2

Small cars          Square, R2, L2, Triangle, Triangle, L2, R2, Square 

TowJam car          Square, Triangle, L1, R1, L2, R2, Square, Triangle

Weird graphics      L1, R1, L2, R2, L1, R1, L2, R2 

Weird sounds        R2, R1, L2, R2, Square, Triangle, L1, R1  

Twinmill 2 Car:
Enter the password TH4T_ROCKS

Thanks to Revolution readers Scott, Martin Brodeur, Jacob Payne, Bryan Hendley, Nathaniel Metz and Pinky878!