From the Super Mario Wiki
When the player decides to stop here, they will meet Bowser and his cohort, Koopa Kid, in Bowser's ultimate bachelor pad. Their quest, Bestest Buds, involves the player getting a necklace, a ring, or a bracelet for Bowser from the Item Shop. If the player gives Bowser the right gift, he'll challenge them to the Bowser minigame Peek-n-Sneak. If the player succeeds at defeating the set number of Koopa Kids, Bowser will reward them with the Gaddget Shroom Bloom.