Firework safety is well worth a reminder - it is as obvious as it is important, but a reminder of the rules is important. Also, revisit concepts of oxidation, rate of reaction and the expansion of hot gases. Choose the level for each key stage. Demonstrations are best, as explanations can be given as pupils watch, hopefully captivated. Try warming potassium chlorate (or nitrate): showing the oxygen, relighting a splint and carefully dropping the splint in (safety screens, and hands kept away!) as you discuss sources of oxygen.
Make some sparklers with magnesium powder and iron filings. Even soak some tissue in sodium nitrate solution, let it dry and show students the "blue touch paper".
For younger pupils, show some flame colours (strontium - red, sodium - orange, and barium - green) by burning their chlorides or mixing carbonate and dilute hydrochloric acid by the Bunsen air-hole. The ammonium dichromate "volcano" will be a hit as the orange powder hisses and seems to grow as well as turn green (make sure you use a fume hood and a small quantity).
Finally, take a look at some of the excellent websites exploring principles of fireworks.