Unlimited turbo:
Press Down(3), L, R, Z at the main menu. The lower left corner of the screen will flash and the message "Need for speed" will appear to confirm correct code entry.

Moon gravity:
Press Left, Right, Up, Down, B(3) at the main menu. The lower left corner of the screen will flash and the message "Feel lighter" will appear to confirm correct code entry.

Bouncy bike:
Press Up(2), Y(2), X(2) at the main menu. The lower left corner of the screen will flash and the message "Bouncy, bouncy" will appear to confirm correct code entry.

Big heads:
Press B, X, R, L, Right at the main menu. The lower left corner of the screen will flash and the message "Your head expands" will appear to confirm correct code entry.

Tiny mode:
Press L, Z, Left, Right, B(2) at the main menu. The lower left corner of the screen will flash and the message "Drink me" will appear to confirm correct code entry.

Tag mode:
Press Z, X, Z, X at the main menu. The lower left corner of the screen will flash and the message "Tag you're it" will appear to confirm correct code entry.

Hint: All bikes:
Successfully complete the hardcore part of career mode.