Sand is your best option in my humble opinion . Hay or straw gets moldy and holds a lot of bad fungus and other diseases , and with sand you can rake the poop out , without having to replace it as often as you might with hay , straw or chips . It's just practical and inexpensive . That's what I use . Put some lime in there also , it will help suppress the odor . Pine needles are great to use in nesting boxes ; they help keep bugs out of the nest . I also use Noalsan or Oxine as a disinfectant , I use a two gallon sprayer and spray the ground , the coop and boxes to kill all bacterial microbes .