Enter the following at the Main Menu...

SD cars:
Hold X and press Right, Right, Up, Up, Down, Left, Down, Left, Right, Up, Down, Left, Right, Up, Right.

Transparent car:
Hold Start and press Up, Up, Right, Left, Down, Down, Left, Up, Right, Right, Up, Down, Left, Left, Down.

Big tracks:
Hold X + Select and press Right, Down, Left, Up, Right, Down, Left, Up, Down, Right, Up, Left, Down, Right, Up.

Wide turns:
Hold Circle and press Down, Down, Left, Down, Down, Down, Left, Down, Up, Down, Right, Left, Right, Left, Up.

Monochrome graphics:
Hold Select and press Right, Right, Right, Right, Up, Up, Up, Up, Down, Right, Up, Down, Right, Up, Left.

Less gravity:
Hold Circle + Start and press Right, Up, Down, Left, Right, Up, Down, Left, Right, Up, Down, Left, Right, Up, Left.

Extra tracks:
Come in first place on every levels in STC and SSS modes to unlock extra tracks