My chickens go nuts for their food as a warm mash. It's a great way to use up the little bits and dust left at the bottom of their bowl. (I feed 20% protein all flock pellets.) It's the same thing as their regular feed, but they go for it like it's candy. Anything extra, ie, treats, I put on top of this.
These are things I give as "ok once a day" treats: greens of any kind, tiny bits of fruit or vegetables.
These are things I give as "once or twice a week" treats: melon or squash guts, including the seeds.
Black oil sunflower seeds ("BOSS") or meal worms are no more than once a week, and tossed out so they have to scratch for them. It's entertainment as much as food. About 1 tablespoon per bird.
Very seldom do I give them bread as a treat. That's an "interactive" treat, ie, they have to get on my lap for a morsel. About half a slice total for the four of them.
I aim for the 10% guideline as well, for anything except the mash.