Bonus Cars:
Level 1: Rocket car by gold house
Level 2: Monorail car on monorail track by station
Level 3: Boat car on the big ship in the harbour (c spanker) in the green box on the pile of green boxes
Level 4: Quad bike in caravan park
Level 5: Monster truck on ledge by the front of stadium
Level 7: RC car on top of krusty burger by the big accident

Show Credits:
At the Options menu, hold L1 + R1, then press X, Square, Square, Triangle.

All Reward Cars:
At the Options menu, hold L1 + R1, then press X, Circle, X, Circle.

Fast Cars:
At the Options menu, hold L1 + R1, then press Square, Square, Square, Square.

Invincible Car:
At the Options menu, hold L1 + R1, then press Triangle, X, Triangle, X.

High Acceleration:
At the Options menu, hold L1 + R1, then press Triangle, Triangle, Triangle, Triangle.

Jump Car with Horn:
At the Options menu, hold L1 + R1, then press Square, Square, Square, Triangle.

Coins list:
Here is a list of how many coins for destroying these things:
Buzz Box: 31
Wasp Camera: 25
Krusty Glass: 2
Lampost, Tree, Bin, etc...:1

One Hit Kills:
At the Options menu, hold L1 + R1, then press Triangle, Triangle, Square, Square.

All Cameras:
At the Options menu, hold L1 + R1, then press Circle, Circle, Circle, X.

Show Speedometer:
At the Options menu, hold L1 + R1, then press Triangle, Triangle, Circle, Square.

Turn Secret Car into Red Brick Car:
At the Options menu, hold L1 + R1, then press Circle, Circle, Triangle, Square.

Grid Mode:
At the Options menu, hold L1 + R1, then press Circle, X, Circle, Triangle.

Trippy Mode:
At the Options menu, hold L1 + R1, then press Triangle, Circle, Triangle, Circle.

Living room variations:
If your internal clock is set to a holiday, the main menu living room will change to accomodate that holiday.