Mine have their food in their feeders free-choice, and a couple of times a day they get scratch sprinkled in their yard to give them something to do.
When they were free ranging, they did not eat nearly as much of their feed, so their 'treat', ie reward for coming when called, was their grain sprinkled on the ground, and a tiny, tiny bit of scratch with it.
On hot days, they get some frozen fruit/veggies or watermelon chunks as a treat, given to them in the shade.
Since my cockeral has been injured, I have been making a mash out of their grain, and mixing some liquid vitamins and yogurt into it. Really only the boy needs it, but he want to be with his flock, so everyone gets it and no harm done.
Non-ration treats easily make up less than 5% of their diet, but they seem to be quite content anyways.