I can't say this enough, put something in the corridors of your dungeons/redoubts/space stations. Far too many RPG adventures reserve the corridors for wandering monsters and the occasional pit trap. You can tell by how seldom one sees keyed corridors on a dungeon map.
It will cut down on the "there's a door there must be an encounter" syndrome. It'll make your adventures more interesting.
Some suggestions for the typical underground fantasy world labyrinth beyond placed monsters:
* Chalk marks showing direction and maybe warnings. These can be in a commonly known argot, simple pictograms or directional arrows.
* A square clearly drawn in chalk, ink or blood on the floor. Just try keeping players away from that or conversely try to get them to step on it.
* Tracks that are easy to notice. There is nothing wrong with using this technique to warn the players about impending dangers or lead them into a death trap. This is an excellent harbinger for
* A wall built up across the corridor. this shouldn't block the corridor and clearly isn't original construction and makes a good spot to post guards for a nearby chamber.
* A pile of dung or garbage. Refuse has to end up somewhere, the contents of the refuse could provide clues as to what us lairing nearby and or dangerous diseases and parasites.
* A Body. (See : http://aeonsnaugauries.blogspot.com/2009/07/well-bob-is-dead.html )
* A stray weapon. Simply a weapon layign in the corridor, mayhaps it is a cursed item or someone dropped it during a fight?
* A tangle of roots. A harmless enough feature most of the time but it can reduce vision or tangle up folks trying to pass through quickly.
* A broken up uneven patch of floor. Unstable footing can prove dangerous and is an even more difficult spot to identify hidden traps.
* Cobwebs. Plain old cobwebs, no one passed this way recently or mayhaps strands of old webs are attracting lots of dust.
* A corridor wall lined with shelf fungus.
* Walls lined with torches, lanterns, sconces lit or unlit. Lit sconces indicating someone nearby is keeping them lit and imagine the reaction if they spontaneously extinguish.
* Scattering Vermin. Cockroaches, mice and centipedes scatter as the noise and lights of the party draw near.
* A barricade. Not a clearly well built wall as above but certainly enough to provide some defense to folks manning the barricade and it will certainly slow an advance. Picture the alarm if the party discoveries themselves coming up on the defenders side of the barricade.
* A spike driven into the wall. Good old iron spike driven into a crack in the wall.
* A rope tied to a spike driven into a wall. There has to be some reason to drive spikes into the wall.
* A puddle on the floor. thsi puddle can be very shallow and harmless, maybe makignthe surface underneath dangerously slippery. If the puddle isn't water the material may be far more dangerous then dirty dungeon water.
* A change in the floor surface, gravel, dirt, mud, sand.
* The ceiling is clearly propped up with extra timbers and beams. This area may tumble at any moment if the supports are disturbed. The timbers and beams could be recent indicating the activity of residents maintaining the dungeon or they could be ancient and mouldering.
* A rivulet of water, sewage or chemical waste runs down the side or middle of the corridor.
* A drain in the floor. This drain could lead to virtually anything but shouldn't' usually be wide enough to allow PC races entry, it most likely leads to a pipe which empties into some sort of nearby cistern.
* Furniture set up in the corridor, table, benches, stools. Could be stacked for storage or recently used by residents.
*Barrels or crates lining the corridor. Unlike video games these shouldn't explode or have anything all that interesting inside that would aid adventurers...at least not all the time.
* Water fountain. Drink it if you dare ! Could be a magical fountain but typically it isn't.
* Potted Plants.
* a beast of burden or incongruous relatively harmless animal just standing there. This animal may flee on first sight, follow the party, be easily captured or be downright grumpy. Some pack animals may have some supplies strapped to their backs.
* A short run of stairs going up or down no more then 2 or 3 steps.
* Bloody hand print on walls. Bloody footprints on floor.
Additional features for modern/futuristic corridors:
* Television screen/monitor mounted in wall or near ceiling. Good place to drop clues or window dressing.
* Mirrored lined section of wall. Nothing to worry about, unless it is a two way mirror.
* Security Cameras. Seemingly dead or certainly active and following the party.
* Cork-board/notice- board mounted on wall. A spot to find clues about the recent residents or the origins of the structure.
* Fire extinguisher/hose or fire axe in recess in wall.
* Flashing lights.
* Warning alarm.
* Hand truck leaning against wall.
* Safety/police tape blocking corridor or laying on the floor.
* Water bubbler.
* Coffee nook. Either fully stocked or a bunch of empty containers and no usable grounds.
* Map of complex. Legible or illegible, defaced or pristine. Might just be a corridor guide which reveals nothing but main corridors and emergency exits.
* Warning/notice sign.
* Leaking ceiling.
* Loose wires. These can lay on the floor, across the floor or hang from the ceiling.
* Exposed ducts.
* Exposed plumbing.
* A pile of garbage bags.
* A bunch of empty plastic bottles.
* Paint cans, tarps and a ladder.
* Hall Lockers. A greet place to find random junk and clothing if it hasn't' been looted before.
* Body outline. Surrounded by chalk or tape indicating foul play under investigation.
* Stained rug.
* A big lump under a rug. Dare anyone look at what is underneath?
* Ceiling tiles overhead are loose or in disarray.
* Grate from air duct lose or on the floor. Is there any indication of the duct being used for covert travel?
* Graffiti on the walls. Warn folks about the nearby gangs or related threats.
* Beer cans and cigarette butts.
* Trip wire running across corridor. could lead to a simple alarm, explosives or nothing at all.
* An old mattress lie son the floor or leaning up against the wall.
Every corridor need not have a special feature but dropping them in now and again makes the runs of corridor more interesting then their width, height and length until the next door is reached.