Using an ethanol fuel cell to power led lights and a fan. Fuel cells can be of several different types and comparing them gives the students some idea of how they could be used in the real world.
What makes ATP, ADP, and AMP so valuable? They are used in DNA and RNA to store and release energy and as signals inside cells. Each day, we recycle kg of these to keep us alive.
Very small-scale extraction of ores. On a wooden spill with iron oxide Fe(III)O and some Sodium Carbonate, and place in a Bunsen Burner. The iron oxide is not magnetic, but the iron produced at the end is magnetic, and the filings can be collected and moved by a magnet.
Inelastic collisions comparing one @pascoscientific smartcart hitting two stacked and two inline. Does it make a difference? Not much since the wheels are nearly frictionless. But it is an interesting experiment to do.
Students remember all the Circle Theorems so that they can solve the A-level problems. Finding the Midpoint of a chord, then finding a perpendicular and another perpendicular, allows the student to find the centre of the circle.
Taking a good look at the insides of an oscilloscope to discover how a Cathode Ray Tube works. Once commonplace, now students don't know what it is, and the technology about how it works, and its history in the discovery of particles.