Play in any costume, even before finishing the 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.
Big head mode
Pull out the Cryptographic Sequencer, then hold LB + RB and rotate the Right Analog-stick clockwise ten times to make Batman's head and all enemy head's larger. Repeat the code to disable its effect.
Nightwing Bundle Pack: Unlockable - Achievements
Campaign Nightwing (25) - Obtain 114 medals on the original Arkham City and Nightwing Bundle Pack campaigns (as Nightwing)
Nightwing Revenge (25) - Obtain 78 medals on the original Arkham City and Nightwing Bundle Pack ranked maps (as Nightwing)
Easter Egg - Scarecrow Room
On a ship near the complete bridge going between the Industrial District and Amusement Mile, there is a tiny raised area on it. If you stop on this little platform and take out your Cryptographic Sequencer, you will start hacking something. This hack is very tough, but after you get it the trapdoor opens. When you go down you'll see several hundred bugs scurrying around and a dead henchman. If you zoom in on the henchman he will move a little bit. To the right of the henchman is a clipboard with Dr. J Crane's name on it, and an order for several thousand of those bugs.