Fast cars:
Hold L + R at the options screen and press X(4). The sound of a horn will confirm correct code entry.

Very fast cars:
Hold L + R at the options screen and press Y(4). The sound of a horn will confirm correct code entry.

All bonus cars:
Hold L + R at the options screen and press A, B, A, B. The sound of a horn will confirm correct code entry. Note: You must have first completed all missions.

Red brick car:
Hold L + R and press B(2), Y, X at the options menu. A sound will confirm correct code entry. The secret vehicles normally found in each level will now be a small red brick car.

Invincibility:
Hold L + R at the options screen and press Y, A, Y, A. The sound of a horn will confirm correct code entry.

Super jumps:
Hold L + R at the options screen and press X(3), Y. The sound of a horn will confirm correct code entry. Sound the horn to jump extra high.

Display speedometer:
Hold L + R at the options screen and press Y(2), B, X. The sound of a horn will confirm correct code entry.

Grid view:
Hold L + R at the options screen and press B, A, B, Y. The sound of a horn will confirm correct code entry.

Alternate views:
Hold L + R at the options screen and press B(3), A. The sound of a horn will confirm correct code entry.

Psychedelic view:
Hold L + R at the options screen and press Y, B, Y, B. The sound of a horn will confirm correct code entry.

Alternate credits audio:
Hold L + R at the options screen and press A, X(2), Y. The sound of a horn will confirm correct code entry.

Drive wrecked car:
Jump out of your car as it blows up. Return to the wreck and press Y. You will be able to drive around in the wreck, with no weeks.

New Year's theme:
Set the system date to January 1st to see various things in the game change to a New Year's theme.

Halloween theme:
Set the system date to October 31st to see various things in the game change to a Halloween theme.

Thanksgiving theme:
Set the system date to the last hursday in November to see various things in the game change to a Thanksgiving theme.

Christmas theme:
Set the system date to December 25th to see various things in the game change to a Christmas theme.

Itchy And Scratchy cartoon:
Collect all cards in the game then go to Lisa's level. Go to the Android's Dungeon. Talk to the Comic Book Guy and buy a ticket for the cartoon. Then, go to the Aztec Theater to watch the cartoon.

Ride an ATV or Quad:
You can find an ATV or Quad to ride at the trailer park in Homer's and Marge's levels. It is between two trailers at the far end.

In Marge's level, go to the trailer park short cut then go near the end of it. You will see a card floating on the top of the trailer. Go on the left side of it and you will see a blue ATV.

Drive the monorail:
In Bart's first level, go to the entrance/exit to the highway. Where there is a split road, go to the right and up the stairs. You will see a monorail. Go to the front car and press Y to drive the first monorail car around.

Drive a monster truck:
When on Apu's level, you can get a monster truck, Go to Springfield Stadium. Directly next to the fountain is a green monster truck.

Secret Car on lv. 6!
EVERYONE!! READ THIS!! I FOUND THE POSITION OF WHERE THE SECRET CAR IS ON Lv. 6

On other sites aswell as this one (the best one) I have seen that people have struggled to find where the searet car is for this lv.

Well as you enter the peir (nearest the casino) keep above the beach and turn left. You will see a building with a buzz cola sign on it. USE THE HORN = JUMP CHAT!! To knock the wall on the top-right of the poster, If you have done this you can see an old 50's style car!