As far as i am aware
most materials are ok... so long as you plasticote them, or similar, after they have been sawn or cut, and fully assembled or any material thats already coated in plastic. I think the ply is a bit iffy unless you coat it afterwards, because (as you said in your first post of the thread) the glue used to bond the ply together is not exactly eddible.
I have seen some cool little houses and stuff built from lego.
How about a timber fort made from applewood chew sticks? would take a while to make but would look realy cool though