To get all bikes, complete the hardcore difficulty in career mode
To unlock Big Head option, at the main menu enter B, X, R, L, Right
To unlock Tiny Mode, at the main menu enter L, Z, Left, Right, B, B
To unlock Tag Mode: at the main menu enter Z, X, Z, X
To unlock Bouncy Bike, at the main menu enter Up, Up, Y, Y, X, X
To unlock Moon Gravity, at the main menu enter Left, Right, Up, Down, B, B, B
To unlock Unlimited Turbo, at the main menu enter Down, Down, Down, L, R, Z