Any (significant) amount of ions in solution will make it
electrolytic - giving it the ability to conduct electricity. Salt does this by dissociating into Na+ and Cl- ions. The problem you're alluding to it the oxidation of the Cl- ions into chlorine gas.
What you want are ions you can have in solution that won't oxidize/reduce themselves before the hydrogen and oxygen.
I'm not EXACTLY sure what ions you could use for this particular purpose, but I can list off some common household products you could add to water, to make the solution electrolytic.
Vinegar - Dissociates into H+ and acetate (CH3COO-) ions
Epsom Salt - gives Mg
2+ and SO
42-Lemon Juice - High in Citric, Ascorbic acids...both produce ions in solution
And the other thing is, just plain tap water is usually pumped with enough minerals and stuff to conduct electricity. That's SUPER easy