To give your rider big tires, pause the game and hold L, then enter the following code: DOWN, B, B, DOWN.
To divide your trick score by 10 (lord knows why), pause the game and hold L, then enter the following code: UP, DOWN, UP, B, B, X.
To make your rider jump higher, pause the game and hold L, then enter the following code: UP, UP, UP, UP.
To give yourself more time, pause the game and hold L, then enter the following code: X, UP, B, A.
To multiply every trick score by 10, pause the game and hold L, then enter the following code: X, B, B, UP, DOWN, DOWN.
To turn on a visible balance meter, pause the game and hold L, then enter the following code: LEFT, B, X, Y, X, B, A.
To slow down time, pause the game and hold L, then enter the following code: X, Y, B, A.
To turn on alternate colors, pause the game and hold L, then enter the following code: DOWN, DOWN, DOWN, DOWN.
To turn on perfect balance (and ruin the game), pause the game and hold L, then enter the following code: X, LEFT, UP RIGHT.
To unlock unlimited special, pause the game and hold L, then enter the following code: LEFT, DOWN, Y, B, UP, LEFT, Y, X.
To unlock the best Tony Hawk level of all time--Burnside--you'll have to do quite a bit of work. With every single character in the game, you'll have to get all 30 magazine Covers and Gold medals in each of the competitions.
Retry a level in Career Mode ten times in a row without completing it to unlock Granny. This will turn your current rider into Granny, and unlock her for career use.
To unlock the Warehouse level from the original Tony Hawk, you'll have to pull off a combo of 200,000 points or more.