Master code:
Press L, Select, A, Select, R, A, L, Select at the title screen to enable cheat mode.

Level select:
Enable the "Master code", then press L or R to select a level. Press Left to highlight "Start" to begin game play at the selected level.

All weapons:
Enable the "Master code", then begin game play on any level. Hold Select and press B during game play to get 99 of each weapon.

Free movement:
Enable the "Master code", then begin game play on any level. Hold Select and press the D-pad during game play to move in any direction.

Lakamul Rain Forest level:
Press A, L, A, R, A, L, Select(2), Start at the title screen.

Nine continues:
Quickly and repeatedly press Start at the continue screen until the remaining number of continues increases to nine.