I've also taken the approach that in nature they would have been exposed to all kinds of different food. So from day 4 or so I've been giving them access to chick grit and been feeding them (not all at the same time!):
1. hard boiled egg with cooked oats -- they wouldn't touch it.
2. sprouts -- they love it, but not at first. It was an acquired taste, and I needed to chop it up some more.
3. mealworms -- need I say more?
4. stinkbugs -- these they figured out by themselves from day 7 or so. Now they catch them and have a ball chasing each other with them. There are so many in the coop that I cannot limit their access to it, but it doesn't seem to harm them.
5. uncooked rolled oats -- reluctant at first but this morning the saucer that had the oats was empty so i guess they decided they liked it.
6. cucumber -- wouldn't touch it. Maybe it was too cold (from the fridge) or not chopped small enough but after a couple of hours I took it out as it was becoming very mushy by them walking on it and standing in it rather than eating it.
I will try hard boiled egg again by itself to see if they like it. I also give them a small handful of hen-treat (mixed grains with some dried veggies, I think) that I scatter on the bedding so that they have something to scratch for. Tonight I will try some clover that is trying to take over the world, starting at our yard. It'd be great if I could feed that to them in any larger quantities next year to help control it.
So far they are healthy, active and very curious to discover what I have in my hands the next time I come into the coop. To the point that they jump on my hands even if I am not holding any food.