I don't purchase special treat foods for my chickens and I don't give them the human food, but in the past week or two they've had:
Garden weeds of every non-toxic variety,
Cantaloupe guts and rinds,
Watermelon rinds,
Pepper cores,
Tomato trimmings,
Lettuce cores,
Carrot and cucumber peels,
A turkey carcass after I'd pulled all the meat I could get from it,
A roast bone after I'd cut all the meat I could get from it,
Half a cabbage that had wilted in the back of the fridge,
Several lettuce plants that had bolted in the garden,
A literal handful of scratch tossed into portions of the bedding that I wanted them to turn over (not more than twice a week),
And a modest amount of Dubia roaches.
Chicken food + water makes a good treat. Chickens seem to really like it.
My adults are completely uninterested in wet mash, but I started giving it to my chicks to help with their pasty-butt problem and they go nuts for it.