To unlock the minigame version of Baby for use in the main game, play each minigame at least once. If you've forgotten which games you've already tried, the game displays a play count on each minigame. After playing each minigame once, you need to play one more minigame of your choice so that the game will save your stats. Baby Robo will now be available in the main game. Note that to select Baby Robo, you have to highlight Baby on the character select screen and press 1.
Play as Grandpa
Successfully complete all stages with all bananas and hidden bananas.
Super Characters
Successfully complete World 1 through World 9 with only one particular monkey (either AiAi, Baby, Doctor, GonGon, MiMi, or YanYan) to unlock the 'Super' version of that character. The 'Super' version of that monkey can accelerate slightly faster.
Unlock Sinking Swamp World
Complete each of the default eight worlds without using any continues to earn a crown in all of them to unlock a ninth world called "Sinking Swamp".
Unlock Staff Credits game.
Simply beat the first World to unlock the Staff Credits game under the Options menu.
Unlock Worlds 9 and 10
In order to unlock Worlds 9 and 10, do the following:
Effect - Code
World 10 - Complete World 9 without continuing.
World 9 - Complete Worlds 1-8 without continuing.
Baby Robo...