Cedar is a definite no-no. It can kill your snake.
I use paper towels for my hatchlings. I used aspen for a little while but man oh man was it hard to find their poop. Paper towels are really good b/c they're so cheap, readily available, and you can spot the poop right away.
I use aspen for my bigger snake because he is a burrower and loves to tunnel all over the place. He likes to lie under the aspen all day and come out at night. Finding the poop in the aspen is not a problem for me with this guy b/c I always make sure he poops outside of the tank on folded newspapers. This way I can go longer without changing the aspen and he doesn't stink up the cage with his poop.