One "treat" that you never have to worry about feeding is their regular feed with a bit of water mixed in to make it a mash. I make mine about the consistency of cookie dough. When the chicks are little, I'll put a bit in the palm of my hand. Eventually, one of them will be brave enough to peck at it.
"Hey, everybody! Look what I found! It's really neat food!" nom nom nom nom.
It's also a great way to use up the bits that end up at the bottom of the bowl, that the birds don't seem to want to eat.
Mash snack is still my chickens' favorite "treat." And they're 2-5 years old.