You could give them a few mealworms or crickets, though those are like the "caviar" of treats 'cuz they get kinda pricey.
I give my chickies squash (any kind will do) and cucumbers. They really like bean spouts too, though those don't have any nutritional value. I give them various kitchen scraps.
The important thing to remember is to give them treats in moderation.
Have fun watching them discover new things to eat. I love watching them peck at a new food item just to watch the reactions. Some chickies love certain things, while others "can't be bothered". I think alot of it has to do with them getting used to the textures.