[Brian]

Two of my favourites; use a pencil to draw a line down the insulator of a sparkplug. Works particularly well on single cyl motorcycles!
A shot of brake fluid up the exhaust pipe will cause untold amounts of white smoke to be emitted for many, many miles. That stuff just doesn't burn away. I like to push a hose up the tailpipe, into the muffler, insert a funnel into the end of hose, pour in a good amount, remove funnel and using your mouth, blow the fluid right up. [yum]
disclaimer; Now that I'm a mature, respectable citizen, I cannot condone such juvenile behaviour!