1) Take a 20 oz Poland Spring bottle, or other lightweight plastic container with a screw-on lid.
2) Using a pocket knife, cut a small (~2 mm diameter) hole in the lid.
3) Put ~5 oz of liquid nitrogen in the bottom of the bottle.
4) Screw on the cap.
5) Shake vigorously once, then quickly place the bottle on the floor on its side.
(This was an improvised demo yesterday, after a late decision to lecture about superconductivity. I had liquid nitrogen for use with the high-Tc supercondustor we use to demonstrate the Meissner effect, but none of the other usual liquid nitrogen props, so I had to come up with something on the fly. This worked pretty well-- the bottle shot along the floor for 10-20 feet in the second or two before all the liquid boiled off.)
(Safety note: You want the hole in the top to be large enough that it's not likely to become plugged up with anything, as that can lead to the bottle exploding, which can be bad. Those angled dropper bottles that chemists use are great fun for this trick, too-- the escaping gas makes them spin like very cold fireworks-- but the dropper tubes are narrow enough they they frequently freeze up with water vapor, and then they explode with a loud bang that can be a little alarming.)
Kewl.
Just make sure that you don't touch the outside bottle in regions where the liquid nitrogen might be floating. Your hand would freeze pretty quickly.
All posts with "low-budget rocketry" in the title should require an attached youtube video.