Sound test:
Press any direction on the D-pad, followed by A, B, X, or Y at the Nickelodeon screen. The sound that is played will vary, depending on which D-pad direction and button were pressed.

Level select:
Go to the trash heap during the second part of level 1. Press L, L + R, R(2), Up, Down, L + R, Select, Start. The sound of a scream will confirm correct code entry.

Make Grumble blink:
Press L or R on the screen that appears before the start of a level.

Make Grumble open his eyes wide:
Hold R + Down when Grumble appears before a level begins or at the options screen. After releasing the buttons, Grumble will appear to be angry.