I've tried: cored apple, watermelon, pumpkin, well cooked quinoa, spinach, chard, soft tomatoes (think squishy, stem cut out), grapes, peppers, cucumber, black soldier fly larvae, and black oil sunflower seeds. All were enjoyed, haven't tried others yet.
Take a bite of it, they'll go nuts wanting it. Also, if it is hard to peck at without it moving, they will struggle to eat it. Weighted or tied down helps. I use an old hand weight and use a hair tie stretched just barely over the weight onto the handle. Then, I put the greens bundled up in to the hair tie so they can peck at it instead of dragging it when trying to eat, followed by giving up.
Mine will eat significantly less if I cut it up further into anything bigger than diced tomatoes. This is probably because it is slightly too big to eat but is hard to break up when because gravity doesn't do enough. So, now I leave treats in one chunk if I can. You can freeze a treat in ice for the same chunk effect. If you just leave it with them, they won't eat it at first, but then it is all of a sudden gone by the end of the day. For the melons, squashes, berries, etc.: they prefer it to be a little overripe but not to the point they're going bad. Mine prefer it as ripe as it goes. Also, birds have acquired taste too, just like other animals. Give them time to get used to something. Everything you mentioned is a solid treat they should enjoy given time (a day or two) to get used to it. Throwing away, or storing in the fridge, what was left keeps your birds safe from potential mold.
One thing I cannot get my chickens to eat is cutting board scraps (except peeled skin strips) I figured it was because of the awkward sizing of it for them. I also tried peeled banana that was too overripe for my liking once, they each tried it and did not ever come back for seconds. I might need to try the banana again because I have another that I won't eat.
