Down The Tubes level:
Pause the game, then press Up, L, Down, A, R, A.
For Pete's Sake level:
Pause the game, then press R, L, R, L, A, R.

Level 5 level:
Pause the game, then press R, L, A, B, B, A, L, R.

Snot A Problem level:
Pause the game, then press R, Up, Select, L, R, Left.

What The Heck level:
Pause the game, then press Select, R, B, Down, L,