Find someone with a TV, the bigger/older it is, the better.
Get some aluminum foil and scotch tape and scissors.
Take a single sheet, preferably large enough to cover most of the TV. Do not tear the sheet with the metal teeth on the box, use scissors.
With the same scissors, bevel the corners a bit.
The key here is no sharp edges. This is because sharp corners can discharge. You want the foil to be as charged as possible to produce as large a shock as possible. You see, the metal teeth create a large amount of sharp edges, which is why you should not use them.
Using one piece of tape per corner, tape the foil to the TV.
Turn the TV off and on again several times. Be careful though, too many times could result in serious injury, and we don't want a lawsuit on our hands, now do we?
What you've made is a giant capacitor between the inside of the TV, and the aluminum foil. Basically, it'll shock stuff.
If the TV is on, turn it off, put down the remote, and leave.
Whoever comes to watch TV next will come in and wonder why there's aluminum foil on the TV. They will proceed to try to remove the aluminum foil and get an large shock.
Alternate ending for Mafia-types - fill the room with some sort of odorless, colorless, flamable gas, the spark from the TV will ignite the gas, and kill the person in a huge ball of fire.