Newspaper, astroturf, aspen shavings. No sand or other substrate made with sand as the snake can become impacted. If you wanna go aesthetically pleasing, take a look at some of the vendors at a local show who should have some nice vines, caves, hides, etc for your snake. I use newspaper and Raydin still manages to get himself underneath to hide after he eats. Put it this way, if your snake burrows, allow him to do so and keep using a substrate that will allow him to do so. Your local pet stores should also have things like cage backgrounds which always add to it. You can buy these per square inch or foot I believe. Measure your cage, take a look at what they have to offer and maybe grab one of those to add a different look to the cage? Also, some people use stuff around the house like toilet paper rolls, twisty ties, yarn, etc to create tunnels or overhanging hides above the ground, almost like a jungle gym. I've seen some cool pics of what people have done on here. Look around at other threads using the search engine at the upper right hand of the screen. Type in "viv" or other key words and see what other ideas you come up with. I'm sure others will chime in soon. Good luck and take pics when you get it all decorated up so we can see. Later bud.
lol