From the Super Mario Wiki
When the player decides to stop here, they will meet Goombob, who is in love. His quest, Accessorize!, involves getting a pretty seashell for him, so that he can give it to his girl, Goombetty. Seashells can only be found at the Sandy Beach in the Seaside Area. If the player returns to the Goombob Manor with the seashell, Goombob will give it to Goombetty, then rewards them with the Gaddget, Dessert Menu.