The first couple weeks, chicks should be fed just their chick crumbles. After than, begin introducing chick grit by sprinkling it on the floor of the brooder. Give them a day of kitting out their little gizzards, and you can very sparingly begin introducing treats such as live meal worms (dried worms are harder to digest), a bit of melon, a lettuce leaf, yogurt, minced boiled egg, or dried rolled oats.
Other than the meal worms, which seem to be in their DNA to go wild over, chicks may be slow to warm up to any new treat. Just give them time.
Be very careful to keep treats to a bare minimum as too much can overwhelm their delicate crops and digestive systems. Warning - this is easy to do when you see the hilarious reaction they will have to meal worms.