There is a safe treat list floating around somewhere. I give mine table scraps, but I make sure I don't give them too much. So far they have had various meats (cut, and cooked thoroughly, chicken, steak, pork, they have had a little hamburger meat), various veggies both cooked and raw (spinach, broccoli, a tomato every now and then, peas, corn on the cob or off, green beans, etc), fruits (watermelon, strawberry, cantaloupe, apples, etc), cooked rice (not something that is the norm, I had some left over and it wasn't enough for another meal, but I didn't want to waste) and cooked noodles, some breads (but I try not to go heavy on them), and that's all I can think of right now. Mine love love love fresh fruits and veggies more than anything, there have been some meats they have turned their nose up at, but they seem to really love chicken. I don't give them anything with bone still attached. I limit their portions. Oh, and they also enjoy clover, grass, and anything else they can get their beaks on outside. I've bought night crawlers several times since there seems to be a shortage of worms available for picking (they're around here somewhere, just not where I can find them lol). Yesterday they were happy to get their beaks on a praying mantis.
Oh, also, I don't give them super fatty meats, dairy, or things that are acidic. I want them happy and healthy. So far, it seems to be working. I have a container I toss some of the leftovers in that's specifically for the chickens, and I put it in the fridge and just add things here and there before bringing it out to them. I wash after every use too.