My first treat at 3 wks was meal worms. I just held one in the palm of my hand where they could see it wiggle and before I knew it one chick came up and snatched it and ran around with it. Barred rocks were the first brave ones, then the buffs, one of the RIR's - had to dangle in front of the other RIR and one of the EE's. My sky is falling Henny Penny EE is too afraid to take one but will grab one if I drop it in the brooder and someone doesn't beat her to it.
Then I fed scrambed eggs in tiny worm like sizes and they loved that as well to the point of flying out on my back, arms, lap to get some.