Get All Bikes:
Go to the bike select screen and press Up, Left, Up, Down, Up, Right, Left, Right and the button for Modifier Airs

Get All Levels:
Pick your rider and bike and at the select screen press Left, Up, Right, Down, Left, Down, Right, Up, Left, and the button for Modifier Airs

Get All Styles:
Go to the style select screen and press Left, Up, Right, Down, Left, Down, Right, Up, Left, and the button for Modifier Airs

Slim Jim character:
If you beat the game with Dave Mirra(or others, I don't know)you get the slim jim character. on the last level when you get the gold it will say "new character!
you unlocked the Slim Jim character". He has solid straight hair and its hilarious!

Secret bike:
If you beat the game with Dave Mirra(or others, Idon't know)you get the slim jim character. on the last level when you get the gold it will say "new bike!
you unlocked the secret bike ". The secret bike has the best stats