
I loved that effect for its entertainment value. I showed the technique to the other kids at the after-school supervision thing. (It was a sort of daycare arrangement, but for middle school kids. I wasn't allowed to be home alone until somewhere around...I don't know, 14 or 15?)
Anyway, there was a plastic slide there in the playground. So, the fun part: Hold a quarter or metal object up to the metal supports at the bottom of the slide, and you could draw out a pretty nice spark. Because the spark didn't hit your skin directly, you didn't get the pain from the heat of the spark, and the tiny bit of charge transfer could happen over a larger area of skin.
The downside: Some people would hog the slide a bit, climbing back up to build a bigger charge for a bigger spark.
Look up triboelectric series. See the list of materials. Choose clothing that is closer on the series to the material the slide is made out of. The farther apart the materials, the greater the degree of charging you're going to get. I.e., there will be a huge difference between wearing clothing made of cotton & clothing made of polyester.
If I were a smart ass, I would suggest covering the bottom of the slide with a sheet of aluminum foil. Then, have the kid wear something that makes a lot of static. Then, rolling some aluminum foil down the top of the slide, then touching the top with one hand and the bottom with the other hand. I'm just joking though - DON'T DO THIS - it will knock you on your ass (or worse.)
Edit: I changed my mind. Do this and post the video. Do *not* allow your kid to do this though.
Damn.
Now I need a playground, and an absence of easily-frightened adults, otherwise someone would probably call the police when they see me putting aluminum foil on the slide.