Successfully complete the game under the hero or super hero difficulty setting to unlock the Green Goblin costume option at the specials menu. Select that option to play as Harry Osborn in the Green Goblin costume with his weapons in an alternate storyline where he tries to correct the Osborn family's reputation.

Alternate Green Goblin costume:
If you are using the Alex Ross Spider-Man, play any level with the Green Goblin in it and he will have an alternate costume that resembles his classic costume more. Note: This will only work when you are in the Alex Ross costume and the Green Goblin alternate costume will not be in the very last in-game intermission.

Play as Peter Parker:
Successfully complete the game under the easy or higher difficulty setting to unlock the Peter Parker costume in the specials menu.

Play as wrestler:
Successfully complete the game under the easy or higher difficulty setting to unlock the wrestler costume in the specials menu.