Had the air spade on 2 red oaks. Looks great, now what can be used to fill in cedar mulch, large pebble stones suggestions?

Do you have a photo? From just your description I'd hesitate to fill back with anything - the whole point of using the air spade was to let the trees breathe by exposing their root flares. If you're simply looking for something on the ground nearby, then a thin layer of mulch would be fine, but don't fill it back in. Mulch will eventually become soil and fill it up again.