To get bonus rounds in Classic version, at the title screen, press L, R, L, R, L, R, Right, Select. Find the door and push it to start the bonus rounds.
To unlock Shoe power-up mode in Classic version, at the title screen, press Select, R, L, Left, Right, R, Select, Right.
To unlock Super Bubble Bobble in Classic version, defeat the Classic version Boss with two players. Then, press Left, R, Left, Select, Left, L, Left, Select at the title screen.