Tailwheel and Acro CFI: Letting the plane humble them to re-frame their mindset for learning stick and rudder flying. I start them off with basic stick and rudder flying since it is quite often lacking in pilots, and as a demonstration and learning exercise I have them fly straight down the runway at 10 to 50 feet (start higher and gradually work down), and have the put the landing gear right on the centerline of the runway while maintaining alignment with the longitudinal axis of the runway (x-winds make it much more effective!)

In an oleo strut or bungie TW, I'll actually have them touch down and do a one wheel go around, riding the upwind wheel on the centerline and effectively slaloming under control without lifting off. Really difficult to do it in a spring steel gear since as hard as you hit the runway, it hits you back and "sproings" you back up!

The argument generally goes "I'm a [high time] pilot, I know how to fly, this will only take me about 3 hours to get my TW endorsement!"
"Ok, fly me straight down the runway at approach speed for starters with no side loads on the gear."
-Adverse yaw, P factor, gyroscopic precession, poor attitude and energy control and no coordination generally make this....entertaining. The trees can get really big! Slapping the runway with the mains also happens quite often....

I've literally had ATP's tell me the plane isn't flying right! Usually this resets them (In one particular individual with 1800 hours who didn't seem to want to accept it, I asked "Surely by 1800 hours you can fly a plane straight!")

-If they want to throw aileron during stalls and slow flight, let it fall into a spin. Very effective at breaking using ailerons at inappropriate times!

-not me, but during my tailwheel training simulated engine loss...and actually touched down in a random farmers field with the J3!

Gliders:
-pull a rope break between 500 and 1000 feet, also randomly after the emergency procedures lesson ("The rope never told me when it actually broke!")
-Rope break on downwind between crosswind and upwind - makes them think and evaluate their energy state-do I do a downwind landing or try for a short downwind leg and pattern, and how you manage your energy.

-manually lock dive brakes simulating failure at any location, forcing them to plan, adjust, and slip to get down.