Programmer high scores:
Press Right(2), Up(2), Down, C at the title screen.
Level select:
Press Up, Down(2), Left, Right, B at the title screen while "Press Start" is blinking. A sound will confirm correct code entry. Note: Only one level may be played if this code is enabled.
Cloud change:
Hold A and press Up, Down(4), Up at the title screen while "Press Start" is blinking. Use the D-pad on controller two to change the appearance of the clouds and other effects.
Suicide:
Pause game play and press A, B, or C.
Invisible Sonic:
Move behind an object and press B.