Interior building project

Hi. I want to make a basic platform in some material that's not to hard to carve in and not poisonous to my frog. I also want to make a background but I'm really not that into building materials and don't know what to buy. The base that will be the foundation in my terrarium will have different carved levels and a shallow pond with some small white led lights making it look nice and clear. In the pond I was thinking about glueing dull rocks with silicon and the base in it self will be covered with husk and need to be light so I can lift it out ad clean it. The background will have plastic plants on it and I want it to look like a solid wall of rock, I might even install a waterfall. This will be my knew art project. I've made models before but not interior to frogs. :/ (sry about my English.. couldn't describe it that good)

Suggestions on easy material and "save" paint?
I'm going to make some sketches in the meanwhile.