During gameplay, press the L Button, Z Button, A and B simultaneously.
At the end of each and every Mini-Game, get enough for a heart fruit and then eat it. After all the letters appear, put them in the right positions, and you'll get a hidden message from Yoshi.
In the Trial Mode, pick the color of Yoshi that you want and press Start and Z at the same time. Then press B to go back and go to the Practice Mode, and you will have the color Yoshi you chose.
On top of the different epilogue messages depending on which levels you finished, there are also 4 different Happy Tree scene endings depending on your mood (the score).
White and Black Yoshis appear as Giant Eggs, but will hatch after completing the level.