@jingle, I'm brand new to chickens, so anybody else eel free to chime in if I'm feeding something I shouldn't - but they have had raw mushroom stems, celery, apples, dry cereal & crumbs, birdseed and bread crusts. They ate everything, definitely went for the mushrooms first and celery last when there was more than one thing tossed out. They seemed to really like the mushrooms, then apple bits (no seeds).
We have a little bucket under the sink and whatever kid crumbs I sweep off the floor plus any veggie stem cuts go into it. We don't have a lot of leftovers or waste, so they don't get alot of food treats. I've been meaning to ask the grocery store if I can pick up the scratch/dent fruit and veggies when I'm in, but haven't yet.