First person mode:
Press Start to pause game play. Then hold L and press Y, B, Y, B.

Turbo mode:
Press Start to pause game play. Then hold L and press Y, B, X(2).

Welsh mode:
Press Start to pause game play. Then hold L and press Y(4).

Enable "Real Controls":
Press Start to pause game play. Then hold L and press Y(3), B.

Squeaky voices:
Press Start to pause game play. Then hold L and press Y, X, Y(2).

Get Turbo:
While in one of the mini games like Twisted System, pause the game and hold Down and the left trigger and press Y, A, A, Y

Its not the greatest code, but I like the game and it helped me out.