I personally like having a hard surface because it's easier to clean, nicer to walk on for me, as well as the animals, and sometimes you don't always want your chickens feet to be in the mud or muck all of the time. Although sometimes when it rains it gets slippery, I dust the pathways with a little dirt for traction if it looks like they're sliding around too much.
Could you add a sand box? Or just put a lump of dirt in a corner, and then they can have a place to dig and scratch. You could also throw hay or grass clippings in a pile and that will be fun for them to scratch around.