Level select cheat: Enter the following code when you get to the trash in the second part of Level 1:
L, L + R, R(x2), Up, Down, L + R, Select, Start.
If you have entered the code right you will hear a scream.
And some fun stuff...
Change Grumble's expression : When Grumble comes on the screen just before you start a new level hold down R + Down for about one second, when you let go his eyes should be wide open, and looking a bit different.
You can also hold down L + R together for one sec in the same place and grumble will blink when you let go.