We once had a really good display dancing about on the windscreens of a 737 and invited the "galley slave" up for a look. She was suitably impressed. Then my first officer decided to show off and pointed his finger at it. There was a loud crack as zillions of excited ions transferred to the end of his outstretched digit. Total loss of street cred on effo's part as he then let out a great yelp, jumped six inches and desperately fanned his smoking pinkie! Stupid boy.
I've never seen it myself, but when St. Elmo's really gets up a head of steam, it can change into the rare form of ball lightning. This truly impressive phenomenon goes bouncing off down the cabin and departs out the back with a suitable loud bang. I'll bet that grabs everyone's attention!