Level Select:
Press Start to pause game play, then press Right, R, B, A, L, L, A, R.

Buttville Level:
Press Start to pause game play, then press L, A, Up, R, A, R, A, Select.

Down The Tubes Level:
Press Start to pause game play, then press Up, L, Down, A, R, A.

For Pete's Sake Level:
Press Start to pause game play, then press R, L, R, L, A, R.

Level 5 Level:
Press Start to pause game play, then press R, L, A, B, B, A, L, R.

Snot A Problem Level:
Press Start to pause game play, then press R, Up, Select, L, R, Left.

What The Heck Level:
Press Start to pause game play, then press Select, R, B, Down, L, B.

Thanks to Revolution reader gamer2101!