- Aluminum soup cans (labels removed)
- Aluminum soda cans
- Glass bottles (wine, vodka, soda bottles, etc)
- Aluminum foil painted in different colors, and either wadded up in unusual shapes, or like sheets of wrinkled paper.
- Old pots and pans (dirtied up to look like they have food residue)
- Random pieces of scrap metal
Splatter lots of acrylic paint around on the trash (browns, reds, yellows, greens) to dirty everything up. And hit some items with washes of black or brown acrylic to really make things look grungy.
Try and paint items individually, and use contrasting colors so the heap has items that stand out in various sections. Green bottle on one side, red scrap of foil on another, blue soda can up front, etc. That way the color contrast makes the heap appear more detailed.
You can fireproof a pile of paper/cardboard, but it would not be cheap. It's usually best to start with fire resistant materials where possible.
If you really want/need items like cardboard boxes, start with a base of non-flammable materiel like the list above, and then fireproof select items and lay them on top. (news paper, pizza boxes, etc) That way you don't have the expense of putting fire retardant on a large pile of stuff. (and it's safer)