Oh you'll get a mixed bag of conflictong answers here. There is a sticky post that lists good treats you can offer them, but it does seem toe a matter of taste or preference to the chickens. You just have to try things out and see what they think.
For my girls, I tried frozen blueberries a few times and they were curious about it but really didn't care for it. Even thawed or mashed. Now I did see them go after one that was sitting out for a couple days.
They love kale. I have it handy for our bearded dragon and they never refuse kale. The store ran out of kale once and I brought home some collard greens. They liked that too, but not as excitedly. May be more fibrous and it does have a strong aroma to it. They kind of like cabbage, but I prefer giving darker greens instead. Not so hot on carrots, but haven't tried it lately.
Gave them seedless grapes cut in half/quarters and chicken keep away was a great show. I only gave them 1 whole grapes each since they found their beaks sticky after running around in circles.