Maybe even Homeland security might be interested which is why I'm not labeling this as a fake (whisper radioactive) substance. Honest folks - it's totally harmless and wouldn't even harm a Kryptonian flea!
Step 1: Ingredients
1) A "glow stick". These are commonly used for party decorations, after dark street fairs, and even practical applications like identifying folks in the dark. On every space shuttle which launched at night or could have a night landing the astronauts had these glow sticks attached to their launch/entry suits.
The same substance is used by plumbers to look for leaks, so if you've got a contact at a plumbing supply company you can get a larger quantity at a reasonable price.
It's an organic substance (the same chemicals which make fireflies glow) and non-toxic.
2) A fancy flask or other clear container.
3) A mark with a sense of humor.
Step 2: Cut Open the Tube
CAREFULLY cut off the end of the tube. Remember that there's broken glass inside from where you broke the inner tube to mix the two chemicals. The warning on my light stick said the chemical could stain surfaces.
Pour the contents of the tube into your flask.
It's not absolutely necessary but if you've got scary music playing, a lightning storm outside, a Jacob's ladder in the background, and cackle with an evil laugh then you'll get style points. Too many mad scientists these days just do the chemistry and forget that style is important.
Step 3: Enjoy!
I would very strongly recommend that you do not put an international nuclear trefoil symbol on the side of your flask and leave it in an airport or drop it off from an unmarked black van without license plates at a hazardous materials amnesty day. Really - don't even think about doing that.
On the other hand, if you happen to know a really dumb super villain who is interested in purchasing Kryptonite and has no way of testing the Kryptonite to verify whether or not it's real, just make sure you accept your payment in cash and make sure the villain has no way of reading this instructable.
Note - this instructable should not be used in any way to harm Superman(tm). It should also not be used to freak out employees of any government agencies with three letter acronyms (TSA, FBI, CIA, NSA, etc.) or anybody else without a sense of humor.
But it would make a great decoration at a party.
The green glow in the photo has been enhanced because my camera doesn't have enough sensitivity to show the glowing liquid.