“There’s something very familiar about all this…”
Early storyboards showed this as a Swatch-branded hoverboard. Other concept art showed amn Airwalk hoverboard, and surfboard-sized hoverboards (which can be seen briefly being carried by an extra early in the film).
In this frame (above) you can clearly see the (green) Velcro used to hold the lightweight foam boards to the actors’ shoes when hanging from the wire rigging to film the ‘hovering’ shots.
This still proves the existence of the rare and elusive orange Mattel hoverboard (undr the other child’s foot):
And I think that’s a CD player on her head.