Wario Land 4

Alter Appearance of Princess Shokora
After you defeat the final Boss, the appearance Princess Shokora will be determined by how many treasure chests you've gotten:
0-1 treasure chests, you'll see Bratty Baby Princess.
2-3 treasure chests, you'll see Hideous Princess.
4-11 treasure chests, you'll see Young Cheeky Princess.
12 treasure chests, you'll see the real Princess Shokora.
(Thanks to ess@010-101.com)

Karaoke Mode
Hold Select + Start, + Up + L + R (in Sound Room, highlight Exit).
(Thanks to purple_plum13@hotmail.com)

Super Hard Mode
Beat the game on Hard Mode.
(Thanks to purple_plum13@hotmail.com)