Comic alternate costume
Gather all Sentinal Techs with a character to access his or her alternate comic costume.

Street alternate costume
Gather all Weapon X files with a character to access his or her alternate street costume.

Danger Room missions
Make your way through the game to access the first three Danger Room missions. Now six more Danger Room missions are available by collecting every Sentinel file and Weapon X file with every character.

Finding Tech Pieces
In order to save time and annoyance, run through the level on novice to find all of the six Tech Pieces, then distribute mutation and save your game. Now go back on the higher levels to obtain the missing mutations, without any worry about racing around trying to locate the pieces.