Aaahh!!! Real Monsters

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.
Information in this section was contributed by Full-Gore.

Level select:
Quickly enter the following code at the title screen. Hold R + Up and press A(7), hold L + Up and press A(8), hold L + R + Up and press B(9), then hold Up and press X. You will be taken to a screen with Oblina to confirm correct code entry. Press Left or Right to cycle through the levels, then press Start to play the selected level.
Information in this section was contributed by meccablue85.

Toggle invincibility:
Press L to enable invincibility and R to disable it at the level selection screen.
Information in this section was contributed by meccablue85.

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.
Information in this section was contributed by Full-Gore.