Endless Fever mode:
Beat all levels two times, all mini-games once, and clear the Music History and Puzzles. The Endless Fever item can be found at the shop for no cost. It allows your Emotion gauge to remain full all the time.
Dash Attack Kassa move:
Beat the game, and you can attack while dashing by pressing Y or B.
Alternate title screen:
The title screen of the game will be day or night, corresponding to your DS time settings.
Collect all the toads in each area to unlock this level.
Title screen mini-game:
Hold R and press Start at the title screen to play a toad pushing mini-game.
Thanks to Revolution Readers <(-_-)> kirby man!, Ashley, and the_guy!