Ok, at the risk of being a bad chicken mommy, I do give my chicks some treats at about 3-4 days old. I always put out a little tray of washed sand/grit first, so they have something to help grind the food, and usually I start with greens chopped up into the teeniest, tiniest, pieces I can chop- I mean microscopic! Sometimes it's cabbage, or lettuce, or mustard greens. One of the easiest things to prepare is raw broccoli- just shave off some of the tops and they fall into super-fine pieces.
I also think it's fun to let them have a taste of the fine-shredded parmesean cheese. Snacks are VERY limited at that age- if I have 10 chicks, I bring out maybe a tablespoon's worth of treats once a day.
It's mostly to get interaction with them and to train them that I am good and not scary. Seems to work for me.