Cypress would be a good choice and gives a natural look, and keeps humidity up. If you don't feel like baking it there's mulch made specifically for reptiles. Stay away from gravel, sand, dirt, and coconut husk (coconut husk is fine to put in a humid hide, but can harbor mites and other parasites so be careful).
Seriosuly, can coconut husk harbor mites and other parasites??????? I just got some of that stuff for my bp for when he goes into shed. Do you happen to know this for a fact, has it happened to you personally?????