Budgie seed mix.
Tiny seeds for them to search, new and interesting. They are round, and chickens seem to love round things, this is why I wanna get more of those little play balls for cats, which are basically a cage with a bell inside.
Throwing them around and shaking them seems to be a fun activity.
Mung beans for making sprouts, cooked chickpeas are a nice treat.
I know most people use shaved carrots in the food, but have you tried whole cooked carrots for them to pick at?
Seeds from bellpeppers and chilli, normally kitchen waste, also seem to be liked, same as cucumbers which have been a bit too long on the vine and developed seeds.
Those mushy, seedy innards are loved by many.
Bushels of parsley might also be an idea.