I did urban bases for my Tau, and honestly you need something to break up the grey. I've used small brass beads from the jewelry section of a craft store to act as spent cases, thrown in some bits from other kits, used plastic i-beams from model train scenery, or even just painted up bits of chopped up sprue to look like garbage. Paperclips can be bent pipes or rebar, extra wires can be downed power lines...just clean out the back of your junk drawer and see what you can do.
If you have some left over vehicle bits, those look good, too. A broken door or lost tire can look good on big bases.
Here's an example - I threw in some bolter cases, some rebar and a space marine helmet.