Invincibility:
After entering the multi-command, pause the game, then press [Jump]. Now you can do anything but die or kill. To get out of this state, press [Shoot].
Level select:
Enter Arlene, Arlene, Arlene as a password. A level selection menu will appear at the top of the screen.
Level - Name - Password
2 - The Curse of Cleofatra - Garfield, Odie, Irma
3 - Count Slobula's Castle - Chicken, Jon, Pooky
4 - The Revenge of Orange Beard 1 - Monster, Arlene, Mouse
5 - The Revenge of Orange Beard 2 - Odie, Pooky, Arlene
6 - Catsablanca - Jon, Irma, Garfield
7 - Alien Landscape - Mouse, Chicken, Monster