Mini-mode
Select championship mode and win the beginner, middle, and high speed classes. An "Extra" option will now be unlocked at the main menu. Select it to race in mini-cars.
Go-Kart Code
Hold the L and R buttons. Then, push right on the D-pad and B simultaneously. You will hear a chime if you entered the code correctly. Go to the main menu, and you will see that you have unlocked the Extra 1 mode.
High speed mode unlocked
Enter the password: 2QjD +D1Q 9Fy2 5B0C.
50% of cars unlocked
Enter the password: /6Kb /xMi irLM +/22
Extra option
Finish in first place in all the races in all four championship classes.