My girls got grass and spring weeds as when I'd only had them for about a week (they were 2 weeks old when I got them). They especially loved to tear apart any dirt on the roots. Not sure what they found on the roots and it made a mess in the stock tank they were in, but they loved any greens I put in. Except dandelions... they didn't like them so much.
Apple cores, fruit peels, zucchini, tomatoes... they'll gobble it all. If their hen mama was taking care of them in a farmyard, she'd be showing them where to find all these treats. However, introduce new foods in moderation as you don't want to cause diarrhea.