Before going to any of the end bosses (Whispy Woods, Kracko, etc), get Crash Power. Wait until the boss's meter is full, then use the Crash power. You will drain 1/2 of his energy meter instantly.
Final Level Secret
At the Last level (7-6) before going into the final door, press up and fly up to the moon and go under it. There will be a secret door there. Then just fall with the full life and one up. There will be the final secret room.
Five Extra Lives
In Rainbow Resort, jump or fly up to the ledge above the entrance. Stand directly over the door, then fly up. You will enter a hidden door. You skip the first three mini-bosses, and after beating the last one (Lion), you will enter a secret room with five 1-ups. Enter the door to your right to exit the level.
Grape Gardens Warp
Go to Grape Gardens: Door 6. Beat the Mike enemy. Do not swallow him. If you go to the next room, swallow the first light enemy you see. Turn around and walk back (not very far), and you will see a door. Go through the door and you will see a button. Press it to make a warp appear in Grape Gardens.
Sound Test Option
To access the SOUND TEST option, beat the 'Extra' game.