In my opinion, the two best things about chickens are watching chicks hatch and giving chickens treats!
Chicks can have treats from day one, but usually it is best to wait until they are a couple weeks old, as they are stronger and more excited to try new things. They will be hesitant at first, but I find most to take to the treats very well, especially if they think you aren't watching. If they seem nervous, make the mommy-chicken call and tap the food with your finger to show them it is safe.
I leave a dish of chopped up grass/clover/dandelion, wheat bread pieces, scrambled eggs, and dry oatmeal on a plate. My chickens adore eggs and bread so these I use to tame them as well.
Mealworms are certainly a favorite, as are small earthworms and other insects such as moths. Watermelon and berries are adored by most chickens!
Remember, treats are healthy, but you don't want to feed so many it leads to digestive upset, diarrhea, or nutritional imbalance. Let them taste all kinds of things, hang them or hide them in places to prevent boredom, use them to tame the chicks by holding them in your hand.
It is a ton of fun to give treats!
Unfortunately, insects and the like which normally dominate a chicks diet, are these days regarded at best as a treat. My birds, which are typical, will invest more effort in the consumption of insects over anything else. For many, the most cost effective insects for use as a treat are mealworms and crickets which are both farm raised and store reasonably well. The freeze dried products will be consumed but not with the same relish as with live examples of the same..
I was wondering, does anyone else have hundreds of mealworms and mealworm beetles in their barn? At night they are crawling all over in the flash light beam and during the day they hide under pieces of wood or metal. You just lift it up and the chickens get a feast!
We used to have them in loft of our seed storage area when we produced our own wheat and oat seed. They were only seasonably available and were regarded as a pest.
We don't grow our own grain, and we've done anything to expand their numbers, though they aren't a pest as there is nothing bad that they are eating besides maybe the hay bedding. Its just weird because they look just like the kind you buy, but they're free!