Sound Test and Gallery
Beat the game tand you'll unlock the moon memorial, (accessible from Earth via CIRCLE) that will allow you to not only hear the game's  music, but view your collection of objects as well.
Play in the credits
After you beat the game, while the credits are rolling you can collect stuff by moving the Katamari around. 
Unlock Shooting Stars
Shooting stars can be unlocked when viewing a particular constellation on the Prince's homeworld (the view constellation option) by meeting the Katamari's diameter within the time limits found below:
Star 1 - Under 1 minute
Star 2 - Under 3 minutes
Star 3 - Under 4 minutes
Star 4 - Under 6 minutes
Star 5 - Under 8 minutes
Star 6 - Under 8 minutes
Star 7 - Under 8 minutes
Star 8 - Under 12 minutes
Star 9 - Under 15 minutes
Moon - Under 20 minutes