99+ Lives
On the second world, there is a level called “Millstone Mayhem”. Enter the level, playing with Diddy. Jump on the tire and go the right until you see the monster. Quickly go back down, jump on the tire and go to the left. You should land on the top of the doorway. When the monster comes down and walks to the doorway, you need to slide down and land on its head, holding Left. The longer you hold it, the more lives you should get.
Code
Another code is merry, it changes the music also music for the music test.
Secret Cave
Go to Kaos Kore (the castle) and make an 8 around the two stones in front of the castle eight times and a secret cave should open in the drain.
The secret world of Krematoa
Swim around the 4 stones in the center of the big map. Start at the top and hold X,A,B and Y buttons. Swim about 4 laps around the stones. A volcano appears.
Lost World
To unlock Lost World, using the jet ski or hovercraft, swim several clockwise circles around four rocks (On the first section of the map) for as many times as you can, until Lost World forms.