As I treat I feed BOSS (black oil sunflower seeds), and kitchen or garden scraps. Occasionally I will do a handful of dried mealworms, shredded beet pulp or feed pellets from our other animals (pigs, goats, horses). During the summer they LOVE watermelon.
For my adult flock I don't feed anything other than layer pellets/mash. I have a bag of seaweed I'll add in a handful to the feed when I remember. As a supplement. That's about it. Not even oyster shell.
Mine love Summer watermelon treats as well! It gets really hot here so I cut the watermelon into sections, freeze it, and give them portions of it.