In "The Great Rift," there is a circular room that has ledges floating in it. If you look down, it continues downward and upward. Carefully jump to the very middle top and you will see an item sitting there. This is the Feel Good costume, and it will be equipped immediately once taken.
Unlockable: New Costumes
Complete the game once to unlock two bonus costumes; one regenerates health and one is powerful armor.