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Beat the game once to unlock the bonus mode. Bonus mode features 50 additional levels for the game, which are harder versions of the normal mode levels.
Collect all 10 red feathers on all 100 stages (bonus stages included), to get the ability to change Ivy's sprite into a dog from the main menu.
Complete the game to get access to the picture book in the main menu, which is the opening and ending of the game available to rewatch.