I think sand is best of those because it wont rot, stink and harbor bacteria like straw, hay, and pine shavings.
Sand can be left permanantly while the other ones mentioned need to be thrown in the trash so they don't attract flys and odor as they rot.
PS OOH i just convinced myself to stop buying pine shavings for the coops too. Yes it looks so nice when it's fresh. But i get so tired of shoveling up wet, poopy pine shavings and hauling it away in the wheel barrel when it gets old! I think it will be so much better to use sand in the coop too as well as in the run.
It is soft like pine shavings, it has pretty colors like pine too yet when it gets wet it will dry out and be totally reusable where as the pine start to smell like hog slop and decay, and attract termites when it is old and full of poop.
I think i will also fill a spray bottle with something like Lemon Fresh Mr Clean and water or Lysol and wash the sand with a deorderizer and disinfectant once it gets old too. Then sprinkle some lime or de That should really make this workable.
I justed cleaned mine out using a rake for the straw and large poos and then hosed everything down(my run in on a slope). But the chicks loved it afterward, it has been really hot and they cooled their toes and bellies. It was cute.
You might try the deep litter method, there are folks that swear by it. For an outside run, some folks throw large, fibrous weeds, twigs from the yard, leaves, garden plants, etc. The moisture stays beneath this mat, the chickens get some good entertainment from scratching for bugs that hide in the litter and it takes a long time to mat down. When it does, they add more. There are experts that say the deep litter method in a coop can provide 100% of the protein needs for chickens. I haven't tried it but I'm thinking about it in one part of my coop area.