Codes:
Enter the following codes as your name in single player mode to get the desired effect:

DUBBED.......Fast Car Damage
FASTBOY......Turbo
BCASTLE......Crazy Collinsons
JUSTFEET.....Race As Tyres Only
ELASTIC......Sky High Tracks
TRIPPY.......Psychedelic Background
LONGLONG.....Very Long Race
MINICARS.....Sky View
TECHLOCK.....Extra Track
LUNAR........Car stays in the air longer
BANGBANG.....New option in Challenge mode
BIGLEY.......All Tracks
MECHANIC.....All Cars

Codes:

Enter PADDED as your name to get Padded Barriers.
Enter DINKYBIT as your name to get the Oulton Park Island Circuit.
Enter PUNCHY as your name to disable Championship Qualification.
Enter CMFORD as your name to get the Ford Mondeo. Enter CMDIF as your name to get all difficulty levels.
Type Elephant at the name screen and your car will be an elephant.