Access Bonus Levels even if you die (Classic Mode)
     Enter the code at the "Classic" mode's title screen:
L, R, L, R, L, R, Right, Select. 
Level 20 will have a door appear. Touch it, to enter the bonus round.

Power Up! Mode (Classic Version)
     At the "Classic" Mode title screen enter:
Select, R, L, Left, Right, R, Select, Right. 
This is the same effect as collecting shoes 
(Move faster, make bubbles faster)

Secret Coke can item (Classic Mode)
     Type  in  I.F  at  the high score table. A chime will confirm the
cheat  has worked. Doing this will sometimes make a coke can appear on
your  next  game  on  floor  1.  collect it and it will start a meteor
shower that will kill on screen enemies.

Super Bubble Bobble (Classic Version)
     Firstly,  beat boss in classic mode with 2 players. Enter code at
"Classic"  mode  title  screen:  Left, R, Left, Select, Left, L, Left,
Select. This unlocks the harder mode of the game.