Enter these into the 'Enter Cheat' screen found in the 'Options' menu.

Extras --> Allows you to play Funky's, Cranky's and Swanky's Mini-Games from Main Menu.
Music --> Music Player (Sound Test)
Harder --> No Midpoint Barrels
Tuffer --> Removes DK and Star-Barrels (only on new game)
Monkey --> Start game with 50 more Lives
Kredits --> View Credits