Since I completely disagree with the rule saying that we can't have floatin' things, Alomancy told me that if I could write a backstory explaining why floating islands can exist the rule could be removed. So here I am :D Bear in mind, this will most likely be shit and totally not logical.
Here goes:
A long time ago, the world was a harsh place. Nearly impossible to live in. Humanity struggled to survive, preying on pigs and fish.
As if this wasn't enough, a rift opened between the world and "The Nether". Located under the "Overworld" it was filled with lava and monstrous creatures. This rift allowed monsters to appear in the Overworld. The brutal tearing in the ground where this rift was located, had a strange effect on the gravity. It allowed certain chunks of land (blown away from the rift and scattered around the continent) to float in air, seemingly with their own gravity. Not only these chunks of land float. If you were to place a box in the air in a certain manner, it would float aswell. The gravity seems only to be inplace when things are moving, if you jump from a mountaintop you would fall to your death. But if you find a way to stay afloat for a while you might be caught in this strange, magical, gravital-defect and stay afloat. Flowing liquid aplies to this aswell, it will flow of the floating object in question and either end up in the ocean or on the land below.
When the rift had been open for 10 years, a magician (a since long lost proffession) found a way to seal the rift between the Overworld and the Nether. He used a spell to summon a stone called Adminium, deep under the earth but above The Nether. Adminium is unbreakable, especially since he summoned 5 layers.
DISCLAIMER: THIS IS NOT REALLY WELL WRITTEN NOR DOES IT HAZ GOTZ GRAMMERZ!!!