You can feed them treats but you have to make sure they have grit available so they can digest the treats. Treats should be kept to a minimum (10% or less of their diet) for any poultry in order to make sure they are getting enough of the proper nutrition.
Short grass clippings are a good treat.
I gave mine dried meal worms and hard boiled eggs as a treat. They still go crazy for meal worms.
If I were to do it again (and that may be soon since my girl is broody), I'd introduce herbs and other greens to them. I'd also ferment their feed from the beginning.
I always had grit available to them as well. I used paper towels in their brooder, I would sprinkle their treats, and grit on top of it.
I live on a sand dune so I use sand for bedding for all of my poultry. The adult birds prefer the grit that surrounds the ant mounds here but the sand itself acts as a very fine grit. Good feed stores will carry "chick grit". If I had to purchase grit for the chicks, I would select something a little coarser than sand.
The hard boiled eggs (specifically the yolks) are a good source for increasing the protein for your poults.