I am very new to chickens, considering I have only had mine for about 5 weeks, but I have been putting them in their run during the day and they are loving it. The problem is I live in Arizona and it is 100 here about everyday and in monsoon (which it is now) it can be very humid. My little chickies really seem to like when I freeze an ear of corn. Shuck it, rinse it, and let some water stay on it before you put it in a ziplock bag to freeze. That is what I did anyway. Then I put it in the run and they really enjoyed the cool treat. They also liked frozen seedless grapes. You can also fill up water bottles, freeze them, and spread them around where the birds hang out so they can lean against them to keep cool. I put one where the chickies were trying out dust bathing.
Since I am so new to chickens I find myself going out there every 15 minutes or so to see how the 'babies' are doing and so far they seem to be handling it better than I am (also being new to Arizona).