Completing the game will unlock a Bonus Room that you can access through the "Save Game" screen. If you go to the jukebox in the Bonus Room, you can also play around with the sound test.
After you beat the game, there is a box in the top right corner when you choose which file you want to use. That will enable you to put these boxes throughout the place (you should probably let someone else do this so it can be more fun). In each of these boxes is a hat. Try and find all the hats!