At the Title screen press L Select A Select R A L Select.
View All Levels
Enable Cheat Mode. Then press Select and R or L to scroll through the levels. Press Left to make the boomerang appear around the word Start.
Enable Cheat Mode. Then during gameplay hold Select and press B.
At the Continue screen press B B B.
Enable Cheat Mode. Then during gameplay hold Select and press the DPad in any direction.
Lakamul Rain Forest level
At the title screen, press A, L, A, R, A, L, Select2, Start.
Quickly and repeatedly press Start at the continue screen until the remaining number of continues increases to nine.