Before doing any demonstration with liquid nitrogen see the
Ping Pong Ball Spinner
materials: a ping pong ball, straight pin, ice chest
Before doing the demo get a ping pong ball. The ones with the patterns on them work best for this. If you only have white ping pong balls use a marker to add some colors or pattern to one. Use a straight pin to poke a tangential hole in the ping pong ball.
With the students tell them that you are going to put the ping pong ball into the nitrogen like you did with the balloons. Take predictions as to what will happen to the ball. Use tongs to submerse the ball in the nitrogen (it will want to float). Hold it under for 30 seconds or so. During this time nitrogen is going into the hole in the ball.
Use the tongs to remove the ball and place it on to a table (it may help to have something to contain the ball and keep it from rolling away). The nitrogen in the ball should heat up and convert to a gas. This expands and is forced out of the hole. Because the hole was poked on a tangent as the gas rushes outward it causes the ball to spin - usually at a high rate of speed. Occasionally the ball needs to be "jump started." You can do this be briefly and gently touching the palm of your hand to the top of the ball. This provides some additional heat to get the ball rolling or rather spinning. This works just like a rocket and occasionally the ball will be lifted into the air by the rocket action (if the hole ends up on the bottom).