Air Hog Zero G micro car. Think “reverse hovercraft” — a fan inside makes it hug the wall or ceiling.

The car is 6″ long and perhaps 3″ wide, and the shipping weight of the whole thing is 2 lbs, so let’s say that 1 lb is for the car. (sorry, no SI units for this. Imperial units given, and this is just a quick ‘n dirty analysis) So you need somewhere around 0.1 psi of pressure differential generated by the fan to have it “stick” to the ceiling.