After collecting a bunch of junk I decided to put it all together in a tastefully junky fashion to make a rubble pile. I used a demo CD from work as the base and started gluing on bits of junk with regular white glue. I used a couple of foam bits, some for rock and some for bushes. There are bits of broken plastic from jar lids, and a metal piece from a tape deck.
Once the junk was arranged like I wanted, I covered the base and some of the junk with sand. Once that was dry I spray-painted everything black. Finally I highlighted parts of the junk with dry-brushing and painting. The bushes were dry-brushed too at this point.
In games the rubble pile can provide cover and the colors and shapes make it visually interesting.