I'm not a big fan of treats, though the kitchen scraps go to my birds daily (just the wife and I, we don't make many scraps, certainly not when you consider the size of the flock).
But in season, they do show a strong preference for grapes (we have wild muscadine), watermelons, cucumbers (I keep forgetting to use them), and any growing squash - pumpkin, summer squash, zucchini - all of which I use as ground cover. Oh, and blackberries, which also grow wild in the pasture (they fight the goats for those).
Late in season, they like sorghum sprays and red sorrel.