Mine is, both for doing and sharing, taking two quartz crystals and banging them together in a dark room, as if trying to make fire…cool “sparks”/ lightshow resulting.
Lots of others I enjoy, but that is just my personal favorite. What’s yours? (and be sure to explain exactly how we, too, can do it!:D)
artificial quicksand in a bucket
When I was at the U of I we had a quicksand tank - 8 feet tall and a couple of feet on a side. I spent a couple of Engineering Open House days in that tank, floating there for hours. Quite relaxing! We also had a small bucket-sized tank that people could step into (we had a set of rubber galoshes onhand)
My current favorite is making ice cream using liquid nitrogen. I made fresh ice cream for ~40-50 people at work in 15 minutes. It looks absolutely Mad Scientist (huge clouds of fog everywhere, you have to wear splash goggles and gloves) and well, it’s ice cream!
My all time favorite was the Tom Corbett Space Cadet “atomic ring”.
It was just a plastic ring with a half-globe clear bead which had a dab of phosphorescent paper under it. Get it “charged up” in daylight, go into a dark closet and peer close into the ring. Looks like shooting stars. Should work with any comparable magnifier.
My favorite in college physics class was when they had a yard wide styrofoam ball and “rolled it uphill” The trick was there was a lead weight at the top that was falling toward the hill. The class roared,
Favorite in chemistry was sodium in water. I thought the instructor was going to die when it splashed all over him.
I am currently in the middle of an ongoing experiment attempting to prolong adolescence beyond all previously known limits. So far things are working just fine.
I like the “liquid/solid” trick using colloidal solids (ie. juggling custard).
Our cub scouts enjoyed making rockets from 35mm film canisters with vinegar and baking powder… ingredients in, lid on, place upside down and “pop!”
One that I want to try is extracting DNA using rubbing alcohol and dish soap.
It’s only DIY if you have a lot of cornstarch and an empty watering trough or long enough bathtub, but I love ye olde “fill up a long tank with cornstarch and water, ditch your shoes, roll up your cuffs, and RUN across the surface.”
The kid’s science center at my university did that at the end of last summer- I got to help mix it and play. I love non-Newtonian fluids.
This kind of thing can be dangerous. A kid at my school mixed vinegar and baking soda in a plastic bottle with a screwcap. It blew up in his face and he spent years in plastic surgery.
My son had a school science fair a while back, and he did his project on carbon dioxide - specifically, dry ice. When we got to the fair, you couldn’t see his booth for all the fathers clustered around it. Most popular demo at the fair - at least among the fathers.
Dry ice is very cool.
Damn cold, actually.
Make Magazine has a project to make a Fire Piston, which demonstrates how a compressed gas gets hot. There’s a cool demonstration of the piston at http://www.makershed.com/ProductDetails.asp?ProductCode=MKBG2
I’ve always enjoyed methane bubbles, it’s a little more involved since you need a tank of methane with a spout. If you can get that, though, you can blow bubbles (using normal solution) and then use a long match/torch/lighter to set make fire bubbles.
Thermite is also fun. Er… I mean… don’t try this at home kids?
Radioactive Northern blots to determine transcriptional levels of unknown putative regulatory RNA genes in Drosophila.
But then I’m a grad student.
Dry Ice, water, and a 2 liter bottle. I know this is dangerous. So do not try it at home.
Our favorite college party trick was the electric pickle. Get an electrical cord w/wall-plug on one end. Strip 1/2" of insulation from the wires at the free end. Twist the strands to stiffen each wire end (if you’re an electrical geek you can tin each wire end with solder), and insert one wire into each end of a medium-sized (4-5" long) pickle. Insert plug into wall. After a second or so the pickle will flicker. For maximum effect, turn the lights out just before plugging in.
Pickle should be on a plate, should be stable, and you should not touch it while current flows. Be sure to not push the wires so far into the pickle that they make contact with each other at the center; you’ll blow a circuit breaker. Flicker-pickle will generate a small amount of stinky steam. If flickering dims/dies down, you can restore activity by wiggling one/both of the wires around; be sure to grab by the insulation, and don’t touch the pickle.
This was so popular that when my friend got married 8 years ago I was called upon to do it at the rehearsal dinner.
My favourite experiment involves beer and potato chips.
Pour beer into glass. It has to be the good sort of beer that has a good tight foamy head. Stir the head with a potato head and the foam deflates. It’s caused by the fat/oil in the chip disrupting the protein lattice of the foam, and it’s the same reason you can’t have a speck of egg yolk in your whites if you want to whip them.
In college, to de-foam the beer bong, they would wipe their forehead with their fingers and then swish their now oily fingers in the foam.
A super simple one if you’re a smoker, just exhale the smoke into a straw into a nice, ice-cooled soda/liquid. The smoke will just lay like a fog at the top of the drink (since it’s cold/heavier than the ambient air.)
The experiment probably makes smoking look way more cool than you want if you’re trying to keep kids from smoking, but even adults are usually suprised it works so well.
Another cool one is taking a half full soda bottle, hold a lighter at the top and release the gas into the bottle. After about 10-15 seconds, take the lighter away, then spark the lighter at the top of the bottle. You’ll have a little blue flame at the top of the bottle. Then squeeze the bottle and watch the little blue flame turn into a 5+ foot gas plume. 
(another one you might not wanna show to young kids.)