Level skip:
Press Left, Up, Left, Up, Start, B, Right at the beginning of a level to jump to the next level. Press A at the beginning of a level that has already been completed to return to the previous level.

Item select:
Enable the "Level skip" code and begin game play at the desired level. Then, press Start and press A to get the shoes; press Start and press B to get slime; press Start and press C to get a sword, eyeballs, or bubbles.