Good thread! I bought a Rowenta recently to replace the one my daughter kidnapped and have been using it with distilled water, but just went to examine it and it says 'anti-calc' so I won't refill it with the same.
For nostalgia purposes and because it 'sparks joy' I also use my mother's method: one of those old-fashioned aluminum sprinkler heads attached to a cork - stuck into a Coke bottle filled with water. Sprinkle sprinkle!