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.
Big Head Mode
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.
Costumes
Choose a saved game, on menu screen press Left, Left, Down, Down, Left, Left, Right, Up, Up, Down. You'll hear a bat flutter sound. Then choose to continue your game and you will come to the costume select screen.
Harley Quinn Pregnancy Test
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.
New Game Plus Mode
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.
Skins and Outfits
There are alternate Skins in Arkham City, allowing players to change the looks of Batman and Robin.
1970s Batman
AU: Preorder at EB Games
UK: Preorder at ShopTo
US: Preorder at Toys 'R' Us
Animated Series
AU: Preorder at JB HiFi
NZ: Preorder at MightyApe
Batman Beyond
AU: Preorder at Game
UK: Preorder at GameStop
NZ: Preorder at MightyApe
The Dark Knight Returns
Comes with the Collector's Edition of Arkham City
Earth One
AU: Preorder at JB HiFi
NZ: Preorder at MightyApe
Sinestro Corps Skin (PS3 Exclusive)
Buy a Copy of Green Lantern on Blu-ray from Best Buy.
Red Robin
UK: Preorder at GAME OR gamestation,
US: Preorder at Best Buy
Taunting
Rotate the Right Analog-stick clockwise repeatedly to perform a taunt. Note: If you execute a taunt before performing a glide kick with Batman or Robin, the view will change to the enemy's perspective.