The Barking Dog experiment and the Rainbow Clock were very popular. But it's hard to beat the surprisingly loud explosion of a nitrogen triiodide explosion when it's touched lightly by a feather.
And no one expects a tiny piece of metal dropped into water to steam, hiss, dance about and eventually explode, but lithium (Li), sodium (Na), potassium (K), rubidium (Rb), and caesium (Cs) do exactly that.