What if the other planets were as close as the moon. -- I wonder if the reason the more distant planets are so much larger is because they are further away from the sun. The planets that are closer to the sun seem to all be much much smaller.
This teacher did this for a preschool lesson, but it can be adapted for older kids. They could drop or open the balloons and observe what happens to the solid liquid and gas inside it. This would help identify the properties of solids, liquids and gases.