Bonus modes
Collect all crystals in the game (it will say 100% on your file at the file selection screen) to unlock Boss Battle (Bosubuchi) and Movie Viewer (Zenbu Miseru) modes.
(Thanks to KingDEDEDE@aol.com)

Extra Star Piece or Treat
Return to the beginning of most levels to find a Star Piece or Treat.
(Thanks to KingDEDEDE@aol.com)

Infinite lives
To get an extra life, get the star at the beginning. Press Start and select ‘Try Again’. Go to the same level and go back to get the star again. Repeat this as many times as needed to collect an unlimited amount of lives.
(Thanks to KingDEDEDE@aol.com)