The rubberized shelf liner is great, but I used and old bath towel (make sure no little strings are on it to tangle in toes or get in their throat), absorbant and good traction too. After about 3 days I use the leaf off my alfalfa hay. I shake it off the stems and fill a bucket to carry it to the brooder. They will eat some and its also good feed and it's a natural deoderizer.
I tried pine shavings but they seem to try to eat it, I'm back to straw now, they seem to scramble around on it fairly well, though it takes more effort to climb in the straw, they don't confuse it with the pine bark/shavings, I don't use alfalfa at all - it's dusty, it molds quickly and it's not all that good for them to eat, I've read it can cause problems eating too much of it, just plain ole straw.
I used a plastic childs pool for a brooder with softwood shavings in the bottom. It worked fine but I had to put an old towel under the litter after a chick slipped a tendon. Pick up all spilled food and they probably won't eat the shavings.