Arena
Beat all previous stages.
Earthbound Reference
When facing the Computer Virus boss (any one of them), it will show windows similar to that of when you are fighting in Earthbound/Mother.
Hint: Copy Vision ability
Locate the large diamond shaped green star in Milky Way Wishes. Approach the star and press the button to enter a level. Complete the level to get the Copy Vision ability.
In-game reset
Hold L + R and press Start.
Invincibility
Pause game play and quickly press R(3), L(3), A, B, A, B, A, B at the start of a level.
Kill monsters in one hit
Press Up, Down, Left, B at the selection screen.
Milky Way Wishes
Beat Great Cave Offensive and Revenge of Meta-Knight.
Reduced damage
Pause game play in any level except for "Milky Way Wishes" and quickly press L, R(2), L, X, A. The majority of opponent attacks will now be half as effective.
Revenge of Meta-Knight
Beat DynaBlade.
Secret Level in Milky Way Wishes
On the level select screen in Milky Way Wishes, move kirby to the green star between Cavios and Skyhigh, and then enter the level. Here you can find the Copy ability.
Sound test
Successfully complete the normal games, then finish the Arena. A sound test option will become available.