Treasure Boxes are scattered throughout the Mushroom World in many differnt games. Some hold the most mundane of items while others hold unique items needed to complete a quest.
In Super Mario RPG, Treasure Boxes hover in the air (just like ? Blocks), and must be hit from below.
They can contain coins, items, mushrooms, weapons, and even monsters.
A golden floating treasure box is in the last area of Bowser's Keep, which contains an infinite amount of small gold coins. (small gold coins are worth 1 coin, whereas large ones are worth 10).