At the title screen:

Original Game: right, left, up, down, up, down, up you can get to the diamond rooms even if you die.

Debug Mode down, up, down, up, right, down, left, down, up, down

Codes in Debug Mode:
* L1 - last level
* R1 - next level
* R2 - Debug Menu:
+ see map number
+ Bub's lives
+ Bob's lives