Staff Credits mini-game:
To unlock the Staff Credits mini-game, beat World 1. It will become available at the Options menu.
World 9:
To unlock World 9, beat all 8 worlds without using any continues.
World 10:
To unlock World 10, beat World 9 without using any continues.
Super characters:
To unlock the Super version of a character, beat Worlds 1-9 with only that character. Super characters can accelerate faster.
Unlock Baby Robo:
To unlock Baby Roo, beat all mini-games and then play any mini-game to auto-save your stats. To play with Baby Roo, highlight Baby at the character selection screen and press 1.