In an outside pen, straw (not hay) is good usually. However, the bottom wood of a pen/house can get awfully greasy/slippery sometimes.

When I keep them inside, in a crate, if the bitch is not a blanket-eater I will use a blanket (not sheet) in the crate. Sheets are slippery, whereas blankets give traction.

I take the top off the crate, lay the blanket down on the bottom and up the sides, and I use a drill to screw-in the blanket up the sides, a screw every 4", so that the pups don't get caught under the blanket. A mother will "dig up" a loose blanket, and pups can get trapped underneath. But a blanket that is spread-out over the bottom, and up the sides, and secured by screws (inside-out), will always provide traction for the pups.

Jack