Most of the time common sense will be your guide on what not to feed the chickens.
IMO always have poultry feed available, limit treats to no more than 10% of their daily intake. Poptarts I would definitely limit
Think fresh fruit, veggies, weeds from the garden, sunflower seeds, kitchen scraps, etc.
I feed an all flock type feed which is in crumble form. Mine absolutely go bonkers when I give them a "wet mash" made from it.
I just mix some of the feed in a bowl, add some chopped apple. grapes or whatever fruit is handy, chop up some kale or greens, occasionally add a small sprinkle of oatmeal, sunflower or even bird seed, add some water, then let them go at it. They think it's a special treat, but it's really their food with a few goodies sprinkled in. Hang some kale, apple(s), cabbage or squash up for them to pick at. Cantaloupe and watermelon are great for warm weather. I grow quite a bit for mine so a trip to the garden makes for a scramble to see what's in the bucket.
I'm sure others will chime in.
Here's a handy treat chart:
https://www.backyardchickens.com/a/chicken-treat-chart-the-best-treats-for-backyard-chickens