Lithium, sodium, and potassium undergo reactions with water.
Two liters of warm water in large pyrex vessel, covered with fine mesh stainless steel screen, is on a stool close by in-floor vent hood. Add a few drops from the phenolphthalein indicator bottle, and a few drops of 1M hydrochloric acid if the warm tap water turns pinkish.
Video camera is clamped to the stool leg, and pointed at the bottom of the beaker. Before class, frame the shot and focus on the center of the beaker.
Using the long forceps, pick out the coil of lithium wire from the mineral oil in the small, covered beaker. Wipe the lithium dry on a paper towel. Drop the lithium wire into the water and observe. Hydrogen and eventually steam are evolved, but the reaction does not ignite. The water goes pink underneath the reaction, as lithium hydroxide is the product turning phenolphthalein to its basic color.
Next, with the forceps, take a pea size lump of sodium metal from the mineral oil in the small beaker. Wipe off the lump on the dry paper towels. Lift the edge of the screen and drop in the sodium metal. Replace the screen and get back.
The sodium will from a hissing ball of molten metal, which bounces around on the surface of the water. The hydrogen from the reaction will ignite and burn with a yellow flame. A second or three after catching fire, the ball of sodium will explode with a bang and a yellow flash, with smoke tendrils rising from the vessel into the mouth of the vent hood.
Finally, take the lump of potassium metal out of the mineral oil, wipe it, and drop it in the water. Replace the screen and get back.
Potassium ignites on contact with the water surface, and burns with a blue flame. It also moves around on the surface, but it has never exploded like the sodium frequently does.
Perhaps it is a matter of scale; the lump size that works safely in this demonstration is pea-sized: 7.5 mm diameter sphere, or 6 mm cube. This size will be flashy and exciting, without unexpected scaling issues.
Eye protection, gloves and a lab coat, with the vent hood running. Keep the fan running for a few minutes after the demonstration to clear the beaker.