But of course, too many treats is not a good thing! Sprinkling some of their regular feed on the floor or ground encourages scratching, a good chicken skill, without giving them treats. (Too many treats means they don't eat enough of their feed, missing out on the carefully-formulated combination of protein, fiber, vitamins, etc. in the feed.)
Some people hang up a cabbage and let the girls peck it to death. Others use a suet cage (like for birdfeeders) and stuff some vegetable or another in it (like lettuce or other greens) and the chickens are challenged by it.
If they have something to roost on in the run, they really like that. Like a log, stump, or even a big rock or two.