Some Ideas(just initial):
a demo of burning gases release from electrolysis could be done
pure water should produce H+ and OH- with the OH- becoming oxygen gas
presence both gases could be shown with a glowing ember
now to show the selective discharge - SO4-2 is the ion I'm thinking about
either a Copper(II)Sulfate solution or a dilute sulfuric acid soln would do the trick
the Cu+2 would go to the cathode (in the copper(II) sulfate soln) coating the pt electrode, but the sulfate ion will not go to the anode - OH- will be produced and 4OH- = O2 + 2 H2O + 4 e-
the same is true with the dilute acid except H+ would be at the cathode
the presence of oxygen can be shown with a glowing ember
NOTE: potential problem - what about the free OH- in the soln, won't both soln produce oxygen gas even if sulfate wasn't selective?
Please remember, I'm just thinking through this problem and trying to come up with some ideas.