Lots of great suggestions here! Everyone at BYC helped me so much the 1st year we had our chickens. We were so afraid the girls were gonna just die in the heat, and someone suggested using empty milk jugs and soda bottles filled with water & frozen, then brought out to the coop for the girls to lay against. It helped my girls tremendously!
I also do as others here have suggested--frozen melon rinds, etc. I will take leftover melon my grandson doesn't eat, but you know how it is... after it's been sitting out on the patio table for 15 minutes, it's had 200 flies on it. Well, I'm certainly not gonna eat it... BUT MY CHICKENS WILL! So I'll freeze it up and save it for hot days when my girls need an extra cool yummy.
I do the same thing with strawberries that are starting to get mushy in the fridge. Freeze 'em up and save them for the chickens on hot days.