Blissful Beach Key
From the Super Mario Wiki
The Blissful Beach Key is an item in Paper Mario: Color Splash. It is used to unlock the gate to an oasis in Bloo Bay Beach. A member of the Five Fun Guys holds the key when they play "Toad Shuffle", and Mario must guess which Toad is holding the key after they shuffle around. However, when Mario chooses the correct Toad, they pass the key off to another Toad. When Mario gives proof that the Five Fun Guys were cheating, he is given the key, in exchange for his silence. Mario can then use the key to gain access to the end of the level.