With shipped chicks, I usually wait until they are a week old, just to make sure they are stabilized and not having any problems. Then I grind or chop things into tiny bits for them, in addition to giving sand or very fine grit.
Since they are growing so fast, I try to give them things that are really good for them. Things that will build a healthier chicken, rather than just something they'll like. I also make sure I'm not unbalancing their diet by feeding a lot of just carbs, for example.