Aspen seems to be the most widely accepted "natural substrate". I also use it because of it's ease of cleaning and attractiveness. However, if you wanted something that was a bit darker in coloration, you could try coconut bark shavings or Eco-Earth.
FWIW--I used the astro-turf type tank liners for about the first month I owned my snake. I chose it because it allowed me to keep a much closer eye on my snakes defecation and to watch for any regurgitation that I may have missed in a more natural substrate like aspen. Once I was able to reasonably establish my snakes character regarding digestion, defecation, and regurgitation, I switched to the more natural-looking aspen shavings.