At the main menu, press Left(2), Down(2), Left(2), Right, Up(2), Down to unlock an option to select any of the bonus costumes. If you entered the code correctly, you will hear a sound when you continue your game. -From: Kevin Yanko
New Game Plus mode
Successfully complete the game on the Normal or Hard difficulty to unlock New Game Plus mode. This mode allows you to keep your previously earned gadgets and experience points. Additionally, there will not be any counter icons on thugs, thugs and Bosses will be more difficult, and thugs will have different configurations.
Note: The alternate ending requires the bonus "Catwoman" downloadable content. In Catwoman's third mission, when you have the option to go left or right with the "loot", go right to view the alternate ending. You will still be able to go backwards and not ruin anything in the main campaign.
Use Batman's Cryptographic Sequencer to go to the following coordinates to intercept secret radio broadcasts that read numbers, make a buzzing sound, and then repeat the numbers again. When the numbers are matched to the corresponding letters of the alphabet, they spell out a hidden message. Note: Some of the secret broadcasts may not be available until after completing the main campaign.
Hidden message 1
Location: X: 700.00, Y: 500.00
Decoded message: "I WILL RETURN BATMAN"
Hidden message 2
Location: X: 500.00, Y: 900.00
Decoded message: "YOU WILL PAY FOR WHAT YOU HAVE DONE TO ME"
Hidden message 3
Location: X: 200.00, Y: 500.00
Decoded message: "FEAR WILL TEAR GOTHAM CITY TO SHREDS"
Easy Augmented Reality Challenge
One of the last four Augmented Reality Challenges appears to be difficult because you are supposed to execute a U-turn during your dive bomb. This can be done more easily by using the grapple boost going past the first checkpoint, then turning around while gliding. You can then glide through all the checkpoints much easier.
Easy high scores with Batman in Combat Challenges
Alternate throwing the Batarang and evading or jumping over the heads of enemies until you get to do a special move (combo 5x or 8x). You can do this five times before it will not add to your multiplier. If it gets blocked, it should not break your combo. It seems throwing Batarangs in succession also works well, as you can throw at least two at a time, and it will not break a combo. Additionally, jump into the middle of enemies, and perform the Bat Swarm.
Harley Quinn pregnancy
Go to the manager's office inside the Steel Mill. On the floor, next to the Harley Quinn costume, there is a positive pregnancy test.
Batman: Arkham City reference in Batman: Arkham Asylum
In the Warden's Office in Batman: Arkham Asylum, go back to the room with the Warden's portrait on the far wall. Turn left as soon as you enter the doorway, and approach the wooden wall. You can destroy it with Explosive Gel. It may take a few tries, but one of the panels on the wooden wall can be destroyed. Beyond the wooden wall is an unmarked room with a blueprint on the wall. It shows plans for the relocation or expansion of Arkham to Gotham City. This is a reference to the setting of Batman: Arkham City.
Killer Croc reference
After you defeat Ra's al Ghul and are leaving the subway tunnels, instead of going the correct way, backtrack until you find a gate blocking your path. You should see a button on the far wall and a gap in the iron bars. Throw the Batarang through the gap to hit the button. There will be a cutscene with Killer Croc. Note: You cannot reach this area again after this point in the game.
During the second visit to the Steelmill, you will see The Joker on the large TV. He will start talking about TV shows, and say "How come it all ended in a church," which is a reference to the TV show, Lost.
Note: To access this secret room, you must have the upgraded Cryptographic Sequencer with the "TYGER" codes installed. Just north of the Industrial District in the Harbord, glide onto the Falcone boat west of the two cranes hanging over the water -- where you do the first set of Augmented Reality challenges. Use the Cryptographic Sequencer to hack the door on the boat. Enter "CityofTerror" as a password to open the hatch on the boat. Inside are a bunch of containers with cockroaches on them and a dead inmate. There is also a shipping invoice for Dr. J. Crane (Scarecrow's real name), hinting that Scarecrow did survive the Killer Croc encounter.
Easy "Bargaining Chip" achievement
Towards the end of the game, you will get the "Heart Of Ice" side mission from Mr. Freeze. You must find Nora Fries to complete the quest. Go to the Industrial District in the south-eastern part of Arkham City. You should see a marked area of Nora's estimated location. There is a well guarded building with a locked front door. When standing in front of this building, fly north, and land on the ship. Use the Freeze Blast on the water to create a sheet of ice. Use the Batclaw to navigate to the destroyable wall, and use the Explosive Gel to get inside the building. Defeat the guards, and return to Mr. Freeze to get the "Bargaining Chip" achievement.
Easy "Catch" achievement
You must throw a remote controlled Batarang at the Mystery Watcher to get the "Catch" achievement. After rescuing Catwoman in the courthouse, exit the courthouse, and you should be able to see the Mystery Watcher across the street, on top of the building to your right. Simply throw the remote controlled Batarang at him, and he will automatically catch it, and you will get the "Catch" achievement. Note: You cannot get the "Catch" achievement after completing the Mystery Watcher side quest.
Easy "Gotham Base Jumper" achievement
Successfully complete the main campaign to gain access to the "Restricted Area" at the center of Arkham City. You will then be able to climb the Wonder Tower, which is the tallest point in Arkham City. Climb it, jump off the tower, and glide towards the Church. You will eventually hit the invisible barrier between Arkham City and the restricted zone, and Batman will automatically turn around. To keep Batman gliding, simply turn around to hit the invisible barrier again and automatically turn around, lifting Batman up to continue his glide. If you keep turning and hitting the barrier, you will always remain in the air, and will get the "Gotham Base Jumper" achievement after one minute