To go to a secret bonus level, enter BCDF or GYLM.
To access a different secret level, listen up. Type in the code to go to level 9 (WBGR) and play through the next 4 levels without letting any of the victims die. As soon as you rescue all of the victims in level 12, you'll automatically go to a bonus level as soon as you walk through the exit door. Also, you don't have to enter the level 9 password, instead you can play all the way through it, but in the long run, it helps.
If you want to hear some cool stuff, listen up. When you start up the game, at the Lucas Arts logo, press "L" or "R" to hear a cool sound. Both of these buttons make a different sound from the other, so try them. If you want to view the credits with beating the game, type in XWJR in the password screen, and you should be able to view them.