People. I do a lot of commercial work, "artist's impression" stuff for buildings, structures, etc. It's amazing how a sterile render can come alive with the addition of a few figures even small ones or alpha planes used for figures. Ditto plants, "formal" and weeds, although these need to be used sparingly otherwise their irregular shape can draw the eye away from the composition as a whole. Other street elements = flags, shop names, glass store fronts, shop displays, signs, house numbers, railings, air vents, grills, drains, overhead cables. If you really want to go nuts with detail then include rain staining (especially from wires), chipped bricks, uneven pavements (sidwalks), broken glass inc. windows, smudgy dirt at ground level and roof levels and litter. Of course, all this depends on the type of street you;re creating. :-)

Cheers,
Mark