Big Heads:
At the Main Menu press B, X, R, L, Right

Bouncy Bike:
Press Up, Up, Y, Y, X, X

For Moon gravity:
Press Left, Right, Up, Down, B, B, B at the main menu. The lower left corner of the screen will flash to confirm correct code entry.

Unlimited turbo:
Press Down, Down, Down, L, R, Z at the main menu. The lower left corner of the screen will flash to confirm correct code entry.
To get all bikes complete hardcore in career mode.

No Boundaries:
At the main menu, enter
L,R,Z,L,R,Z
It should give you no boundaries, enter it again to turn it off

Tag Mode:
Press Z, X, Z, X

Tiny Mode:
Press L, Z, Left, Right, B, B