Successfully complete the indicated task to unlock the corresponding costume:
Golden Age suit: Save Metropolis from the tornadoes
Pod suit: Defeat Bizarro
Breaking the sound barrier
While flying as Superman at hyper speed, he goes through stages where after an amount of time of continuous flying he gains a burst in speed. He accelerates and becomes faster and faster. If you fly for long enough (flying a lap or two around the entire city above the sea is recommended), he eventually breaks the sound barrier. It is about the fourth jump in speed. You will know this has happened when the sound of him ripping through the air is suddenly silent. This is because Superman is traveling faster than his own sound.
Developer memorial
In Research Park, by the Superman statue, there is a memorial to Derek Allman, one of the developers. The game is dedicated to the memory of him as said in the credits.
Play as Bizarro
Pause the game and press Up, Right, Down, Right, Up, Left, Down, Right, Up to unlock Bizarro. If you entered the code correctly, you will hear the sound of an explosion.
Trophies
Successfully complete the indicated task to unlock the corresponding trophy:
Bizzaro Unchampion: Successfully complete all Bizzaro mini-games
Globe Bowling: Pick up the Daily Planet globe
Kitten Finder Extraordinaire: Rescue all 100 kittens
Meteor Mayhem: Successfully complete Level 1
Outstanding Valet: Pick up 500 cars
Photo Finish: Defeat 500 opponents
Unlockables
Pause the game, and go into the options menu. This is called the Fortress of Solitude, enter these codes anywhere in the menu. Try to enter them at a fairly quick pace.
Effect - Code
All Unlockables - Left, Up, Right, Down, Circle, Square, Circle, Up, Right, Square Unlock
Infinite Metro Health - Circle, Right, Circle, Right Up, Left, Right, Circle
Unlock All Power-ups - Left, Circle, Right, Square, Down, Circle, Up, Down, Square, Circle, Square