Stop by a green house or the lawn and garden section of a local chain store and look at their plastic pots.  Depending on what scale and genre you deal with there are lots of different uses for them.  They come in hundreds of sizes and shapes and are normally pretty cheap.  Some even have a useful texture molded into them.

The seed starting trays make good industrial complexes for smaller scale sci-fi games.  The larger pots work well for towers and what not throughout various periods.  You can also use the smaller seed trays for making tank traps for more modern conflicts.