Crates, garbage, garbage bins, empty liquor bottles, broken glass, plastic, oil spills, puddles, dirt, grime, grease, ratty blankets, cardboard, homeless people, rats, cockroaches, spiders, ants, bird that eat refuse (magpies, pigeons, etc), street cats or dogs, mice, employees on smoke breaks, broken & discarded furniture, dead tires, graffiti, vomit, mold, mildew, newspapers/leaflets, grimy barred windows and doorways, chain link fences, flickering streetlamps, broken lights, brick walls, peeling paint, cracked pavement, pot holes, small signs on doors with the business name on it, signs that say "loading area", "Private" or "No loitering", parked or broken down cars, crime (muggings, drunken brawls, murders, break ins)
Wind shuffling trash into corners, dogs rooting through garbage, cats meowing, people coughing/talking in low voices or snoring if the alley is inhabited, music from clubs with back entrances, the clink of bottles, a trash bin lid slamming down, the crinkle of a trash bag as it's emptied into a bin, garbage lids being knocked to the ground, the jingle of keys as a door is locked/unlocked, shouting, glass shattering, the clatter of chain link fence as someone climbs over it to escape, the sputter of a car being turned over, chatter/footsteps & car noise from the adjoining street, far-off wailing of a siren, overheard arguments from building tenants in residential areas, music/TV/laughter/arguing from open windows, groaning as someone gets sick, shuffling footsteps through alley debris, the rattling wheels of a homeless person's shopping cart, feet scuffling down steps, swearing as bouncers throw someone out of a club or drinking establishment
Rotting garbage, body odor, animal and human waste, motor oil, cooking smells drifting from open windows or restaurants, wet cardboard, mildew, vomit, 'beer smell' from broken bottles, cigarette smoke & butts, moldy furniture, car exhaust
Bagged lunch from shelters, leftovers from restaurant bins, alcohol, cigarettes, vomit
The rough bricks beneath the hand, using the wall to steady one's walk, falling in a pile of garbage from drunkenness, grime sticking to the shoes, litter crunching underfoot, the cold metal of a garbage bin lid, forcing a heavy garbage bin open, rattling a discarded bottle to see if it has any alcohol in it, the sudden click of a switchblade opening, the prod of a gun or blade during a mugging, pain at being pushed, shoved, punched, kicked or otherwise beaten, tripping against debris left in the alley, sifting through garbage for something usable, accidentally stepping in a cold oily pool of water, slipping in motor oil, metal wire digging into fingers as you climb a chain link fence, shattering a window with an elbow, using a shoulder against a door to break in, ripping a pull handle or door knob, leaning against the wall, sleeping on a musty & legless couch
--The words you choose can convey atmosphere and mood.
Example 1: The wind was howling like a pack of arctic wolves, but hunkered down under his newspaper-and-scraps blanket, Alfred could barely feel it. The bricks at his back were warm from the ovens on the other side. He took a deep breath, smelling the fresh-baked scent and hardly any dumpster at all. Pulling his rough cap down over his eyes, he burrowed into his warm corner with a smile.
Example 2: Matilda gave the doorstep a final, crisp sweep, then looked around in satisfaction. The dumpster was still dingy but sat straight in its corner, where it should have been from the beginning. The wet smell of whitewashed walls assured her that, for now, her alley was graffiti-free. Something skittered in the corner. Matilda cocked her broom and advanced on the sound, eyes cutting to the rat holes, but the concrete she'd filled them with still held. Ah...a rogue leaf. She dropped it in the dumpster and swept her way back inside, shutting the door behind her with the confidence of a job well done.
--Similes and metaphors create strong imagery when used sparingly.
Example 1: (Simile) The smell in the alley was overwhelming, like a thousand cats had come to this specific place to do their business.
Example 2: (Metaphor) The alley was a black hole into which every piece of trash, every sinister sound, every dead end had been sucked.