[Easter Egg] Hello from Kirby!
From the title screen hold L + R + Z and push the A button. This will make Kirby say hi!

Dark Star
The mysteries of the Dark Matter are unraveled on Dark Star.
To access the final level, defeat Miracle Matter at the end of Ripple Star and get 100% of the crystals.

Unlimited Lives
Once you've beaten a level, all of the Crystal Shards will turn into Blue Stars. Go back to a previous level and collect the Blue Stars. Once you have one, pause the game and select "Try Again." Then go back to the level and repeat the process until you've gotten enough lives.

Boss Battle Mode & Movie Viewer
Collect all crystals in the game (the number on your file select screen will 100%) and you will be able to check out 2 secret modes: Boss Battle (Bosubuchi) and Movie Viewer (Zenbu Miseru). You can fight all the bosses in the game in Boss Battle. The Movie Viewer lets you watch all the cutscenes in the game in order.

Minigame Difficulty Levels
Description: Unlocking varying minigame difficulties is simple enough.

How to unlock: Beat the difficulty level before the difficulty level you're looking to unlock in order for the higher difficulty level to become available. So, for instance, if you beat the easy difficulty level, then normal difficulty will become unlocked for the minigame in question. Defeating the normal difficulty will then subsequently unlock Hard, and so on and so forth.