I started giving my chicks treats at about 1 week old. The first was a slice of tomatoe, which they loved. I put it on a paper towel and set it in the brooder box and they went after it right away. There is alot of water in tomatoes so I figured it couldn't hurt them. The next thing I tried was craisins that I had cut in very small pieces. They went nuts over those too. Now they are almost 6 weeks old and I give them lettuce, squash, mushrooms, sliced oranges, melon, apples, avocados, raisins, grass clippings, bread, frozen raspberries, cucumbers, etc. The only thing I would not give them is potatoe peelings and any form of chicken. I have given them hard cooked eggs. I do not buy any of the above at full price; however, I do go to our local food distribution every two weeks at 6:00 a.m. and help unload a truck full of nearly expired food and then buy a share for $14 just so that I can have 'fresh' food to feed my chickens. I also give them whatever table scraps we have left, which isn't many---too many big eaters at my house.
They come running when they see me coming with their treats!