My chickens love watermelon too. It's pretty funny to watch the pullets who have never had watermelon before. They kind of look at it sideways, take a careful bite, and then go nuts trying to get some without getting pecked by the hens.
My chickens love: Egg, bread, cheese, cabbage, whole salads gone bad in the fridge (not moldy or totally rotten), peach, apple, clover, yogurt, and any type of meat.