If you look closely in one of Deadshot's storage closets, you'll see that he has a lightsaber (an apparent reference to the fact that Mark Hamill, the voice actor for the Joker, also voiced Luke Skywalker in the original Star Wars trilogy).
After you fight the "Joker" in the Steel Mill area, you'll need to return to retrieve the cure. Enter Detective Mode when you do. You should notice that the Joker's corpse doesn't actually have a skeleton, just like Clayface didn't when you defeated him in Arkham Asylum. That's some really cool foreshadowing that gives players a glimpse of what to expect at the end of the game, if they're paying close attention.
Using the below cheat, you can select any alternative Batman skin and play them in the main game, without having to complete the full game.
To unlock this, simply press the following combination on the Main Menu screen (after you have selected your save slot):
Left, Left, Down, Down, Left, Left, Right, Up, Up, Down.
You will get an audio confirmation and when you continue your game, you will get the option to select your favorite skin.
Use the Cryptographic Sequencer, hold down all shoulder buttons (don't let go), rotate the left stick counter-clockwise and the right stick clockwise. After a few seconds the mode will activate.
This can be activated at anytime during gameplay "without" the Cryptographic Sequencer as long as you complete the game on Normal. It can be activated in Riddler's Revenge as well and New Game Plus at anytime.
When you get to the mission when you have to get the cure back from Joker you get to a part where a video plays and he rants about The ending to the show LOST.
Here are the dates you can visit Calendar man for a special message. On each day he describes a murder and crosses the day off the big calender to his left. You can make this faster by just disconnecting from Xbox Live and setting your date and time to the correct dates.
In Two Face's courthouse, if you enter through the back door and go downstairs, Calendar Man is locked in a cell, and you can interact with him like many other villains locked up across the city. But with one difference. If your system clock is set to a holiday, Calendar Man will have a speech about that holiday when you interact with him. To see all the dates he has speeches for, simply look to the right of his cell. Against the wall is a calendar with all of the dates circled in red.
Follow this video to find Scarecrow's mask and hints toward his future plans for Gotham City:
In the sewers underneath Arkham City, you can have a brief encounter with Killer Croc! Watch the video for more!
If you use the cryptographic sequencer and tune in anywhere in the general range of 700.00/500.00, you will hear a disembodied voice repeating a sequence of numbers over and over. Every time the sequence ends a tone chimes, and then the number sequence starts over again. The numbers are 9 23 9 12 12 18 5 20 21 18 14 2 1 20 13 1 14. If you assign a letter to each number (A=1 B=2 C=3 etc.), the message translates to "I WILL RETURN BATMAN."
On station 500.00/900.00 you get a different numeric string:
5 15 9 7 21 18 18 14 3 5 24 15 12 7 22 3 10 5 15 9 22 3 8 25 26 15 16 25 10 15 17 25 *music chime* repeats. This can be decoded as "YOU WILL PAY FOR WHAT YOU HAVE DONE TO ME."
In order to unlock the alternate ending, you must have the Catwoman DLC. In her third mission when you have the option to go left or right with the "loot," head right for the alternate ending. Don't worry; you'll be able to "rewind" back and not ruin anything in the main game.
You can watch it by clicking the following link if you wish (SPOILERS!)!
Here's how to get the Story Teller Achievement/Trophy: http://www.ign.com/videos/2011/10/18/batman-arkham-city-storyteller-achievement-trophy
There are alternate Skins in Arkham City, allowing players to change the looks of Batman and Robin.
The Dark Knight Returns
Sinestro Corps Skin (PS3 Exclusive)
Complete Batman: Arkham City on Normal or Hard difficulty to unlock New Game + Mode where you will retain all of your gadgets and XP. Several things are different in the New Game +:
In the Warden's Office, head 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 this with Explosive Gel. It may take a bit of trial and error, but one panel can be blown away. Beyond this 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 nod to the setting of Batman: Arkham City. For video and images of this Easter Egg, check out IGN's article, Warden Sharp's Plans For Arkham City