Select a saved game then on the 'Menu' screen press Left, Left, Down, Down, Left, Left, Right, Up, Up, Down. If the code has been entered correctly you will hear a bat flutter sound as confirmation. If you now choose to 'Continue' your game you will be taken to the 'Select' screen.
To access this alternate ending requires the bonus 'Catwoman' downloadable content. When you reach Catwoman's third mission where you are given the option to go left or right with the 'loot', choose to go right to view the alternate ending. You will still be able to go backwards and not ruin anything in the main campaign.
This feature will become available when you complete the game on the Normal or Hard difficulty setting. In New Game Plus mode you keep your previously earned gadgets and experience points. There will not be any counter icons on thugs and the thugs and Bosses will be more difficult to deafeat. They will also have different configurations.
You can use alternate skins in the main story by downloading the free Batman Inc. Skin, selecting your save slot and then entering the code Left(2), Down(2), Left(2), Right, Up(2), Down at the 'Main' menu.
Search the Manager's Office in the Steel Mill and you will find Harley Quinn's attire from Batman: Arkham Asylum. Next to it on the floor you will see a pregnancy test box that says 'Fruitful Endeavour' and right next to that is the actual test that reads positive.
To enable this feature where ALL the characters heads become large you have to take out your cryptographic sequencer and hold LT and RT. Then rotate the left analog clockwise while simultaneously rotating the right analog counterclockwise.
This mode which allows you to keep your previously obtained gadgets and Experience points becomes available when you complete the game under the Normal or Hard difficulty setting. In New Game Plus mode there will not be any counter icons on thugs and they will have different configurations and be tough to defeat from the start of the game. The Bosses will also be tougher.