By Jesse C Cohoon
1) It's a trap. The exit seals and the following happens (roll a d8)
A) The room fills up with [d4] a) oil b) sand c) poisonous gas d) water
B) nozzles come out of the wall and spew fire to all the characters.
C) shards of glass or metal spheres that combine to become golem
D) The floor drops out beneath the characters, and they fall into a pit with [d4] a) spikes, b) snakes or other small dangerous creatures c) some sort of harmful substance such as a contact poison, mold or jelly d) nothing - just falling damage
E) The ceiling/ walls starts to collapse in on the characters inside or the room itself is a mimic
F) A monster is summoned that must be defeated in order for them to escape.
G) The room becomes a slide and takes the characters to another place in the dungeon
H) The room separates the characters from each other, giving different challenges to each.
2) Secret passage.. Where the passage leads is up to the DM
3) it's a library. Roll on the following table: [d12]
A) Arcana (ancient mysteries, magic traditions, arcane symbols, cryptic phrases, constructs, dragons, magical beasts)
B) Architecture and engineering (buildings, aqueducts, bridges, fortifications)
C) Dungeoneering (aberrations, caverns, oozes, spelunking)
D) Geography (lands, terrain, climate, people)
E) History (royalty, wars, colonies, migrations, founding of cities)
F) Local (legends, personalities, inhabitants, laws, customs, traditions, humanoids)
G) Nature (animals, fey, giants, monstrous humanoids, plants, seasons and cycles, weather, vermin)
H) Nobility and royalty (lineages, heraldry, family trees, mottoes, personalities)
I) Religion (gods and goddesses, mythic history, ecclesiastic tradition, holy symbols, undead)
J) The planes (the Inner Planes, the Outer Planes, the Astral Plane, the Ethereal Plane, outsiders, elementals, magic related to the planes)
K) Erotica - books about sex
L) Races - useful information about different races in the game.
4) a thick layer of dust that causes the characters to start to sneeze and wheeze, causing penalties for attacking and defense
5) a garden. If underground, consider how the vegetation is getting enough light to grow. If above ground, consider it being a solarium, or greenhouse.
6) A zoo. Captured creatures are in cages, a good many of them obviously dangerous.
7) A sanctuary of some kind, provides travelers with [d6] a} safety, as per the Secure Shelter spell b) those inside the room don't attract notice of wandering monsters c) accelerated healing: all normal damage is healed d) bonuses: characters taking refuge inside, grants a bonus for X number of hours after having left e) divine relief: all penalties disappear after spending X number of hours inside f) divine relief 2: all diseases, maladies, and curses are gone after spending X number of hours inside the sanctuary.
8) Interesting features [d4] a} Natural: stalagmites, stalactites, other stone features b) interesting architecture, possibly of a lost era c) visual effects: the room acts as an optical illusion d)interesting but dangerous [d4]  ice  lava  poisonous swampland/ stagnant water  fire
9) common household items [d6] a) bedroom set (bed, table, lamp, comfortable chairs) b) a kitchen/ dining room c) a bath/ swimming pool/ steam room, etc d) an office (desk, chairs, some books, if in a modern era, a computer, printer, etc) e) living room (couches, seats, TV [if in modern era]) f) oddhouse/ bathroom
10) a fighting arena/ gymnasium: this could be as simple as a boxing ring, all the way up to free weights/ machines designed to strengthen the users.
11) Another dimension or time.The doorway is a portal to another dimension. Other rules of gravity, magic, etc may apply. Or a doorway into another time (past or future)
12) Teleport. The doorway teleports the characters, causing them to be someplace else entirely. Is what the characters see the actual place it transports them to, or is it an illusion?