Bubbling Magnesium

When magnesium ribbon is placed in hydrochloric acid, hydrogen gas is produced causing the mixture to bubble.
Mg (s) + 2HCl (aq) → MgCl2 (aq) + H2 (g)

If they hydrogen gas is collected, it can be ignited to demonstrate the potential safety concerns of an evolved gas.
H2 (g) + O2 (g) → H2O (g)

Hydrogen Balloons

hydrogen balloon explosion

This demo shows the explosiveness of hydrogen, by igniting a balloon filled with hydrogen.
2H2 (g) + O2 → 2H2O (g)

Relight A Candle

Candle wax is flammable as a vapor, so a candle can be relit by igniting the smoke trail of a recently blown out candle, which contains wax vapor. This can be used to demonstrate how caution needs to be taken with flammable vapors.

Aluminium Foil and NaOH

aluminum foil and sodium hydroxide produce hydrogen gas

This demo simulates what happens when Drano is poured down a clogged drain; the hydrogen gas produced forces gunk out of the drain. Also, this demonstration is a good safety example of why bases need to be handled carefully – they can be corrosive, especially to metals. If the hydrogen gas is collected and ignited, this demo shows the safety hazards that evolved gases can have.
2Al + 2NaOH + 6H2O → 2NaAl(OH)4 + 3H2