another good material is to go for pink foambord from home depot! it comes in various thicknesses, is fairly cheap $$$-wise and once coated in plaster it's fething durable!
this is what we used for pretty much all the sculpted tables at the canaidan hq battle bunker!
the advantage of this stuff is 1" thick sheets you can get - perfect for making things like rubble piles and barricades!
smacking the stuff with a hammer to make rockey outcrops is also a great way to relieve any strees and/or pent-up anger!
cheers!