Donald Duck in Maui Mallard — Cheat codes

Debug Mode
Enter the first password at the password screen, then press Start. Then go back to the password screen again and enter the second one, and press Start. A sound effect will confirm success and Debug will be added as the third option on the title screen.

Debug Mode    IMCARY MAUIMM

Level Select Password (South American Version)
The passwords differ from European to South American version.Here follows the codes for the latter version:

Mojo Stronghold    NOCHNC
Muddrake Mayhem    MUDPIE
Ninja Training Grounds    ININJA
The Flying Duckman    ARRGGH
The Realm of the Dead    DKMPOZ
The Sacrifice of Maui    HTFOOT
The Test of Duckhood    ZEFROG

Passwords

Level 02    ININJA
Level 03    ILVMUD
Level 04    ITSHOT
Level 05    GETHIM
Level 06    YOHOHO
Level 07    UNDEAD
Level 08    GDLUCK

Easter Eggs
Turning Into Ninja to not making Maui dizzy
In order to not make Maui dizzy, you must collect Yin-Yangs.
During The Ninja Training Grounds Level, you must duck by pressing Down and while pressing Down, press and hold the powerup button and wait for few seconds. after few seconds, It will turn into ninja.

BONUS: To turn back to normal, use the same button to turn back to normal form.

Secrets
More yin yang for you
If you have no yin yangs at all and find two or more of these, don't get them all at once. Get only one them and transform. You'll see the yin yang you just got renew itself. Now, get them all and you'll be off with more yin yangs than you would have if you got them all in the first place.