I'm always on the lookout for interesting packing materials, nicely shaped cans, boxes, decent corrugated card - anything that I might be able to use in one of my terrain projects!
Before gluing the pieces to their separate bases/additional pieces, I used some junk materials (in this case, some polystyrene foam packing that I couldn't really use due to it's lack of durability, plus some scraps of foamboard) to add some sturdiness to the pieces, rather than having them hollow. As always with terrain, the hot glue gun is your friend!
Here's one of the pieces, combined with an old junked Rhino, various additions and some polyfilla (spackle for those in the US!) - these could work well in a lot of different urban environments - for example, as pre-fabricated dwellings.