Each randomly generated world comes with at least one floating island. These contain a chest with unique loot and are interlaced with gold and silver ore. Finding a floating island can be tough as the altitude depends on the size of the world you created, but building a "sky bridge" can help. On small worlds, building the bridge at about 200 blocks above sea level will put you within sight of an island. Keep an eye out for vines floating in the middle of the sky, because that's the bottom of your island.