Oh, I see!
Any untreated wood (like pine) should be just fine. However, you could also consider just using a cardboard box which would require less work and be easy to replace.
My indoor rabbits like to lay where they can see what is going on. They don't really like to hide in a house for sleeping. But maybe they were just funny (or overly curious). Right now I have an upper shelf in their cage and that is where they sleep.