These are arenas, hidden items, and other stuff found in challenge mode once again.

Unlockable	How to unlock
Brush	After you have beaten bowser jr at yoshis stadium, jump in the pipe as mario, fight yellow shy guy, then open the chest with wario.
Daisys cruiser pass	Found in the peaches ice garden item shop.
Fire ball error item	Found in the chest behind waluigi.
Luigis flashlight	Found in the mario stadium item shop (note, this will not be useful til after u beat bowser jrs team at bowser jrs playroom.
Mini boos	Mario and wario is needed - Go through the pipe on the pier as mario, then open the chest with wario after a battle with dry bones.
Pow block error item	Found in the chest blocked by baby dasit.
Rattle	Wario is needed - After you have talked to baby daisy, go bak to the fountain and search the right side with wario.
Secret shop pass	Found in the treasure chest behind brown kritter.
Toy field pass	Obtained after wiggler joins you.