We like using coconut fiber. Our lay box is also the humid/cool hide. We find it works well for their multi-purpose hide. Also if a gecko happens to go potty in there, it is easy to scoop out the clump.
The sand will not kill her. But it is not ideal for egg laying. I use the Zilla Beaked moss, some times labeled as Terrarium moss. My Girls love it and it doubles as the humidity chamber.