For the last couple of years I've spent a lot of time hanging around in playgrounds and sand pits. These are of filled with toys, and quite often you also find bits of broken toys, usually from that plastic truck you slowly see disintegrating over the course of a season.
I have been collecting these broken off bits, and here are heaps for my space junk yard, made up of these bits. So, not only do I get to collect terrain bits, I clean up my child's environment too!
Next we have a fuel station for spaceships, made from the pistol grip of a waterpistol. The pistol itself is destined to become a space ship one of these days.