The chickens will eat anything in the run, so something planted for them will soon be gone. I like river sand. It is a bit larger grain than what we normally think of as sand. It is cheaper than other sand. I put about 4 inches in the run. They turn it over into the existing soil, so it is getting to be time for some more.
Sand will allow the run to drain, so that the chickens aren't in the mud. The turning of the soil in the run allows the poop to be buried in the turning process and break down into the soil.
The chickens will dig in whatever there is in the run. If there is enough soil, they will make their own dust baths. You can take a tub or something of that nature, fill it with soil, mix in some wood ashes, diatomaceous earth, cheap potting soil, or such to make it loose. They will dig and flop in it.
Chickens appear to really enjoy dust baths. Mine have dug several holes around the area for dust bathing. Dust bathing helps them keep clean, distribute oil to their feathers, and manage pests.