Unlock Baby Robo:
To unlock the minigame version of Baby for use in the main game, play each minigame at least once. The game displays a play count on each minigame if you can't remember which ones you've played. After playing each once, you will need to play one more of your choice so that the game will save your stats. Baby Robo will now be available in the main game. To select Baby Robo, you have to highlight Baby on the character select screen and press 1.
Staff Credits mini-game:
Beat World 1 to unlock an option for the Staff Credits mini-game in the Options menu.
Beat all 8 worlds without using a continue to unlock the 'sinking swamp' world.
Beat World 9 without using a continue.
Complete World 1 through World 9 using only one monkey to unlock their "Super" version. The "Super" version of that monkey can accelerate faster.
Thanks to Revolution Readers Amy and Shinto!