Access Bonus levels:
At the classic mode title screen, press L, R, L, R, L, R, Right, Select. Level 20 will now have a door in it. Touch it go start the bonus rounds.
At the classic mode title screen, press Select, R, L ,Left , Right, R, Select, Right.
Super Bubble Bobble:
To unlock the a harder difficulty in classic mode, you first must beat the last boss using 2 players. Then at the classic mode title screen, press Left, R, Left, Select, Left, L, Left, Select to unlock the harder version.
Beat all 100 levels in the new version of the game, and he should appear at the character select screen. He's faster, but can't jump as high.
Using Robolun, beat all 100 levels in the new version of the game again. This will unlock Lovelun, who shoots bubbles farther and faster than normal.
Thanks to Revolution Reader Nerdlun!