Flame Test - Ethanol solutions of metal salts are sprayed into a burner flame producing brilliant fireballs with the characteristic color of each metal. This demo is usually used when discussing atomic structure to illustrate that electron transitions
Hydrogen Balloon - In addition to its obvious crowd appeal, detonating a hydrogen balloon can be used to demonstrate combination, redox, formation, and exothermic reactions and activation energy.
Thermite - spectacular reaction iron oxide and aluminum undergo a single replacement reaction producing great quantities of heat and a stream of molten iron and aluminum oxide. Can be used to demonstrate an exothermic, single replacement, or redox reaction.
Limiting Reactant - This demonstration is usually used in a General Chemistry course when the concept of limiting reactant is being introduced.
Iron and Oxygen
This is a very versatile demonstration. It is an example of a formation reaction, a redox reaction, and a combustion reaction.