Extra levels:
Press L, UP, R, DOWN at the title screen. A creature will appear
in the bottom right of the screen to confirm correct code entry.
Select puzzle mode, then choose the new "Another World" option.

Play as Bub:
Press Z + L + R + A at the game selection screen.

Level skip:
Press Z + L + R + A during game play.

Bonus characters:
Select "Puzzle Mode". Then, press LEFT(2), UP, DOWN, L, R, L, R, L + R
at the puzzle selection screen. Bub, Bob, and the Boss characters may
be selected