From the Super Mario Wiki
When players decide to stop here, they will meet Bowser and his cohort, Koopa Kid. Their quest, Boss Bowser, involves a card game, in which players must guess if the number on the face-down card on the right is higher or lower than the number on the card on the left. They must do this three times, with the right card being used as the new left card each time. After that, Bowser will challenge players to the Bowser mini-game Koopa Kappa. If players succeed at defeating the set number of Koopa Kids, Bowser will reward them with two Gaddgets, Faux Flame and Hourglass.