Safe natural looking substrate?

[jovamabob]

Im making a new 2-tier vivarium for my adult from a bookcase. My question is, what substrate can i use?

I want to have a kind of digging area and a non-substrate area in both tiers. Both sections will have hides in, and the hides will be filled with shredded kitchen roll for warmth/comfort. I've been using shredded kitchen roll because aspen made him sneeze. I want the substrate to look as natural as possible though because the kitchen roll doesn't look the best. Since i can't really use aspen again, is repti-bark or Coco-husk safe?