At the Cerebro Files menu, press Right(2), Left(2), Down, Up, Down, Up, Enter to unlock the bonus Iceman Danger Room missions.
Nightcrawler Danger Room missions
At the Cerebro Files menu, press Up(2), Down(2), Left, Right, Left, Right, Enter to unlock the bonus Nightcrawler Danger Room missions.
Wolverine Danger Room missions
At the Cerebro Files menu, press Down(2), Up(2), Right, Left, Right, Left, Enter to unlock the bonus Wolverine Danger Room missions.
Danger Room missions
Successfully complete an Act with a character to unlock one of their Danger Room mission. Successfully complete all three Acts with a character to unlock all three of their bonus Danger Room missions.
Bonus costumes
Collect all Weapon X files with a character to unlock their street costume. Collect all Sentinal Techs with a character to unlock their comic costume.
Finding Tech Pieces
To save time and frustration looking for Tech Pieces, go through the level on novice to find all six Tech Pieces, then distribute mutation and save the game. Go back on the higher levels to get the missing mutations, without worrying about racing around trying to find the pieces (especially on timed missions).