Mine love string cheese - oddly enough so does my cockatiel and I think they would kill me over a cup of meal worms.
Melons. (watermelons, asian melons, american muskmelons, european cantaloupes - doesn't matter the type) If they're homegrown with lots of compost, they're the best. The seeds act as a good de-worming agent, the flesh is refreshing, delicious, and always fun to watch them eat, and the rind is mostly where several different minerals and vitamins are stored in. And with the fact that the melons are not storebought, I can completely trust myself or my birds eating the rind! Jelly melons are the best though - They have a higher mineral content, are the coolest looking in appearance, and the texture of the flesh is a hit amongst the chickens.
mealworms!!! runner up : the inards of green grapes- im too scared to give them whole or even half'd so i hold a half and each chicken helps to clean out the "inards" and when they are done i left with a hollowed out grape skin.
I've heard watermelon but none of my animals like it- the rabbits will eat it if its their ONLY treat. Mine won't eat bread either - picky stinkers!! They do eat rice though And we recently discovered asparagus tops were pretty good!!