Try watermelon, cantaloupe, or kiwi. My chickens go nuts for it on hot, summer days. I am actually more inclined to give them additional fruits and veggies in the winter because the ground is frozen and there's not much to forage for. I scatter the food all around so at least they're occupied...
I feed our flock their regular 16% layer feed, but I supplement their diet with fresh fruits, veggies, herbs, peanut butter, honey, really anything healthy except citrus fruits, raw potatoes, and tomato. Citrus fruits have too many conflicting articles to be worth the risk with so many other...