I've not raised chooks via brooder or separate from parents but in general I would think any natural stimuli you can add will help... Anything that triggers or allows natural behavior. Sunshine is great for chicks and it's good for them to learn to cope with grass etc, otherwise adults who never have seen grass before can kill themselves eating pieces that are way too long. Dirt to dig and dustbathe in, grass or other greens, objects to perch on and bugs to hunt, etc are all helpful... Yours don't sound too far removed from all natural instinct so I think you'll be fine, hopefully. If you freerange them in future you'll be headed back to pretty normal chooks in no time. Some people add dirt to their brooders to help the chicks become immune to whatever the flock or ground at their place may carry. If you do this, be ready to deal with anything they catch. They'll catch it sooner or later anyway, better sooner, and better to be exposed in small amounts so they gradually build immunity rather than get overcome.
One of my hens, the most instinctive one, would always take offense at food being given to babies. She would cover it quickly with dirt, silently, before they noticed, then dig it up and act all excited as if she'd just found it, calling them to come see quickly. Only once she'd hidden it and 'discovered' it were they allowed to eat it. If you hide some treats in dirt, that will trigger some natural instincts too. Even shell grits are good for that.
One thing I would recommend for a few reasons is to teach them to come to you for food. I just say 'here, chook chook chook' --- sounds kinda like what a mother hen says, really, or a rooster. I teach my chooks to come when called, it's useful for a lot of things like rounding up the flock into the cage in the middle of the day. Babies who were never called for food can become mothers who don't tell their babies they've found food, or roosters who don't feed hens, just jump on them without warning or any previous relationship established, and this is stressful and potentially harmful as well. Babies in particular can fail to thrive if they're with a mother who has no instinct to call them for feed. They are either programmed to try everything to see what is food, i.e. hatchery stock, or to eat what is offered to them with the food-call being made. If they've got the instinct to depend on a mother and she fails to deliver, it's not too good for the chicks.
I think the biggest issues occur when babies and adults never see each other, no hen is allowed to brood, no rooster is allowed to see hens and vice versa, (i.e when artificial insemination is the rule not the exception), and also when the sexes are kept separate as a rule. Then you get hens and roosters who kill babies, roosters who fight or harm hens because they no longer know what a hen is, or worse, they're sexually attracted to humans, etc. Social instinct breakdown and confusion is responsible for the majority of violence and stress in domestic chooks, I think, barring cases where the severely restricting environment causes more stress than the chook's fellow inmates.
As you've noticed, your chicks follow the most instinctive; I have noticed all my dominant males and females are the most instinctive. Males and females are mostly irresistibly attracted to the most instinctive member of the opposite sex, there doesn't appear to be anything more attractive to them in my experience. It's a good survival instinct.
But then they fixed up. 
