Ice cubes float in fruit juice as evidence that cool things float to the surface. Being nothing but frozen water they coolify all things that surround them. The magnanimous nature of ice cubes keeps them light hearted and this buoyancy raises them to the surface.
Of course, that doesn't explain Fruit Loops.
Fruit Loops float in milk simply because they are shaped like lifesavers with a hole in the middle. The force of gravity pushes down in the hole thereby elevating the cereal surrounding it. It's a pressure thing.
Unless you're a bubble. Then things are different...
Bubbles float in water for a very good reason. They aren't water. They hate water. And while both are made up of oxygen, the two are more like distant cousins that don't like each other. Bubbles can hang out with water but they make sure they're on top.
So if you float it's for one of three reasons: 1) You're cool, 2) You have a hole in the middle, or 3) You like it on top.