Treats deviate the nutrition they NEED from their feed ... Should never be more than 10% but I've learned they really don't need "treats". @5 Points Wingfeather is right, dried mealworms are a good sparingly given treat expecially when they molt because of the extra protein that helps the new feather growth.
Some believe in treats & table scrapes but I've learned the hard way not always good. Years ago I stopped "healthy" treats all together, my girls were not putting on weight and worms weren't the issue. It was recommended I stop treats ... Only feed, they put on weight and looked happier. Still toss a handful of MW at close up.
During the summer I'll hang an apple or partial small cabbage for extra hydration. I have some strawberry plants for them but the slugs have been enjoying them. I have some Kale growing and will toss 3 leaves in someday, although the slugs took care of them too. Any ideas for slugs? I toss slug bait but that never stopped the slugs. I don't feed my girls slugs as we have Rat Worm Lung rampant here.