Idea
Description: unlock Idea (a hidden character)
How to unlock: complete the game's prologue.

Palette Swaps
Description: unlock up to ten different palette swaps for your character
How to unlock: complete the game once through.

More Missions
Description: unlock three extra missions for subsequent playthroughs
How to unlock: complete the game once through.

More Difficulty Levels
Description: unlock two more difficulty levels ("Expert" and "Master")
How to unlock: beat the game once through.