Skip to Buttville: Pause the game and enter L, A, UP, R, A, R, A, SELECT
Skip to Down the Tubes: Pause the game and enter UP, L, DOWN, A, R, A
Skip to For Pete's Sake: Pause the game and enter R, L, R, L, A, R
Skip to Level 5: Pause the game and enter R, L, A, B, B, A, L, R
Skip to Snot a Problem: Pause the game and enter R, UP, SELECT, L, R, LEFT
Skip to What the Heck: Pause the game and enter SELECT, R, B, DOWN, L, B

Codes thanks to IGN