Don't let ballet flats scare you. I will admit they're seriously uncomfortable, though. No arch support in most of them.
If you can't find a minimalist shoe, find a thematic shoe for an earthbender. Look at the style of dress they wear and the shoes the non-benders wear and go for that, instead. I personally enjoy a matching shoe over a minimalist one, in cases like this. Too many minimalist styles are too modern for a costume like this (my tastes only, of course).
And as for whether it's allowed or not, I wouldn't go barefoot in a renfaire and I have calluses thick as shoe leather, for all the reasons listed above.