The first few things that come off the top of my head...

A couple of fun things that we do at work;

Diet coke & mentos/diet lemonade & polos.. I swear there is not a person alive who doesn't love exploding fizzy drinks with sweets! Best of all, now the science behind it has been properly explained and you can do experiments with different types of sweets/drinks, even using sand in place of mentos to prove your point, to explain how it works.

Potassium permanganate & glycerol. Dry powder & liquid = FIRE!!

Hydrogen peroxide & sodium iodide and a dash of washing up liquid- foams up like crazy and gives off heat.

Cornflour explosion, always good, or cornflour and water for a sticky lesson in non-newtonian fluids!

Ok, that's just a few for now, I also make a mean PVA slime.

>Cornflour explosion
Wasn't that what those terrorist tried to use, but ended up with a floury mess coming out of their back packs?

I think the exploding fizzy drinks with sweets is a definite!

Might not involve any explosions but how about the experiment where you use a lighted match to suck a hard-boiled egg in to a milk bottle?

It's good, but it's better when you use meths.