Blow up the balloon, most but not all the way. Tie the balloon.
Dip a pin or needle in some cooking oil. Poke the pin into the balloon, into the end opposite the knot. Remove the pin.
Hold the knot and twirl the balloon around, so the penny goes to the bottom. Have the penny cover the hole made by the pin.
Turn the balloon over and observe the penny - it "sticks" to the top of the balloon.
By inflating the balloon, you increased the air pressure. The force of those air molecules is greater than the force of gravity, so the penny stays where it is. (It doesn't take much of a force from your finger to overcome the force of those air molecules!)