Shadows of the Empire

Easy - Leebo Scanner:
Hold down the camera button in the Doom sections for five seconds to reveal a 'map' view
that lets you see enemies and items.

Medium - Different Ships:
Hold down the camera button on the final level (while you are flying around Space Hook) and your ship will change into an X-Wing. Hold it again and you'll become a Tie-Fighter!! The ships handle differently, and have different weapons.

Hard - Invincibility and Infinite Weapons:
Hold down the crouch button on the Doom levels for fifteen seconds and D ash will gain invincibility and a 100 of each weapon.