You can easily make your own frozen treats that they love. Buy any sort of produce when it's cheap and freeze it. I cut cucumbers in to four pieces; quarter watermelon or cantaloup; freeze kiwi, grapes, corn on the cob and such whole. It takes them longer to eat it and cools them off, too. Mine will absolutely devour it. We recently gave them frozen watermelon and they ate it so far down that the skin was paper-thin. You could see the sunlight through it.
You can also take greens, like strawberry leaves or pumpkin seeds, and put them in ice cube trays. Fill them half-full of water and freeze it. Fill it the rest of the way and freeze again. That will keep the leaves in the middle of the ice cube, rather than just floating to the top.
You can be as creative as you want. My flock absolutely loves it and it's hysterical watching them peck at the frozen grapes, which then roll away from them! It keeps them pre-occupied, too, so they don't peck on each other as much.