Lots of things! And its so much fun!
My chicken love watermelon, cherries, wheat bread, white bread, cat kibble, wet cat food, oatmeal, corn, squash, noodles, millet, sunflower seeds, beetles and worms, slugs, scrambled eggs (and, unfortunately, raw eggs too), and even meat (they steal that from the cats).
Other treats that people give chickens is plain yogurt, tons of other kinds of fruits, greens, lots of other kinds of vegetables, sorghum, other kinds of seeds and insects, treats of muffin, ect, ect, ect!
I hope this helps. Best of luck!