What to do about overheating?

I give mine frozen watermelon. They love it not frozen, and in NJ we had brutal temps last week so I froze it and they cleaned it to the rind! I have heard but not tried frozen cantaloupe and peas. Or put treats in water in a cardboard 1/2 gallon container and freeze it and they can peck at the treats frozen inside. Mine love grapes too but I have not frozen them.

Right now I have my five chickens in two different setups.  I have a very small coop with tin roof.  I insulated the roof with some of the blue foam board and it makes a huge difference.  I have an old refrigerator drawer full of sand on the floor of the run.  I put large chunks of ice in it and two of mine love to sit in it to cool off.  I also put ice in their water as you mentioned.

The chickens in a shaded pen -- I am misting down the floor of it during the hottest part of the day and putting ice on the ground.  They also have a fan on them.

We are almost finished building our roofed run.  We used to have a greenhouse, but had to leave it when we moved.  I took the equipment out of it, though.  One of the items was a PVC pipe misting system.  I am planning to install it in the run which has a sand floor.

My chickens are currently housed in my sister-in-law's garage which has a dog door to an attached dog run as we have had to evacuate our property due to the Waldo Canyon Fire.  I put two gallon jugs of water in the freezer last night and just now set them on the floor hoping they might lay down near the jugs and enjoy the cool.  I'll let you know if it seems to work.  Other than that, we have a fan going in the garage, trying to offer fresh cool water every few hours and hoping for rain.