Have you tried making a wet mash with some of the normal complete chicken food? Just add water, and they think it is a great treat.
Since it is a properly balanced food, you don't have to worry about them missing getting full of "treat" and missing some important nutrient.
It can be warm in winter and cool in summer, just by using different temperatures of water.
If you need to lure the chickens somewhere (like back into the pen after free ranging), mash is nice because they cannot pick it up and run away with it (I had that trouble when I tried to use bread crusts as a lure-- they picked up the bread crusts and ran back out.)
It is easy to make any amount you need. They get just as excited about a few bites each as they do about a full bowl of it. The less you use, the more they think it is a "treat," and the more useful it is when you want to lure them somewhere or make them excited. If you leave it available all the time, they will start viewing it the same way they view their normal dry food: something to eat in the right quantity, but no need to rush and gobble it up.
If you ever have a sick chicken that doesn't feel like eating, wet mash will often encourage her to eat more than dry food does.
As you can probably tell, I think wet mash is great stuff for chickens