Me and my younger brother had a way of crashing each other's adventures by going the wrong way. He was literal and logical, and I'm me. So, either we'd solve each others' missions easily, or wind up on the wrong edge of the map, until a friend told me how he just dropped the dungeon he had prepared wherever the players wandered. So I started to create multiple dungeons, and while the campaign-specific ones were located where they needed to be, if the party went the wrong way there was an adventure there. Once the adventure happened, that location was permanently on the map.
Some ideas I used:
Goblin Village:
In a gully a band of goblins, (or level-appropriate humanoids,) have created a camouflaged village. By tenting over the natural crevice they gain concealment, and they have dugouts, similarly tented, to provide living spaces, store rooms, and defenses.
This village is protected by trapping the approaches, and by tunnels leading to duck blinds from which guards can observe and use crossbows from concealment and cover. They can also trigger traps manually rather than risk a good rogue spotting the tripwire. Multiple choke points control the ability of invaders to get in, while multiple concealed egresses allow the villagers to flee as the warriors heroically die defending them.
This can be a one-night adventure or it can be the prelude to dealing with an invasion. The complexity of the village should be adjusted by level and player skill. Having it as a hunting camp on the fringes of a goblin kingdom came to mind, but in my campaign world, goblins were never a major plot element.
The Broken Tower:
What was once the stone keep of a motte and bailey fort now stands as the only surviving structure in a log palisade long gone to rot. At night a ghostly light shimmers from its upper windows.
A handful of weak skeletons in rotting leather armor bearing rusty iron swords may be encountered in the area. Entering the keep itself requires accessing the third floor entrance which was once accessed via a now rotten wooden bridge from the manor, which has collapsed into rubble. There are crosslettes at that height, and narrow windows on the next two levels up. What might have been a conical wooden roof on top has collapsed.
Entering the tower at the third floor or higher, one discovers the place is filled with armed and armored skeletons. Additionally, skeletal wizards are there. (Adjust their competence to make the encounters level appropriate.)
Rooms like pie sections connect to one another on each floor, and a stair around a central column spirals between the three floors. (In mine I had a barracks and storehouse on the lowest floor, officer's quarters and a wizard's library on the fourth, and a lord's family quarters on top.) There are no voids in the first and second level, apparently to provide strength against siege weaponry.) At night, the ghost of Lord Myrhh prowls the uppermost floor. (Scale to level-appropriate incorporeal undead.)
The central column is hollow, and a ladder in it goes down through the entire structure to a basement inside the motte. There are storage-rooms filled with rotten provisions, but there are also sealed doors into catacombs. Within the catacombs are treasures protected by skeletons, zombies, mummies, and such. A ghoul cleric has a shrine there as well. (Scale to the adventurers, including other spellcasting ghouls and ghoul minions.)
The Mad Dwarf's Cave:
Mik was once a renowned metalsmith, whose many creations still Garner a great price in the rare opportunities that they come up for sale. However, as some dwarves get gold-fever and become reclusive hoarders, Mik became obsessed with adamantine. Some decades ago he vanished while supposedly questing for a source of adamantine ore.
The adventurers come across a tailings pile on a hillside, perhaps filling a gully or streambed and forming a pond. The natural vegetation has grown over it, but the oldest trees will be less than ten years old. At the hillside end of the tailings pile is an horizontal mineshaft blocked with boulders. Clearing the boulders and entering, one discovers a small maze of mine tunnels and shafts.
There are strange little tripedal, three-armed, three-eyed creatures with a three-jawed mouth atop their heads. They use these jaws to eat rocks they find, and they burrow mouth-first into the walls, ceilings, and floors, at about two body-lengths per minute. They are not hostile, but they crave metal, and will try to eat metallic adventuring gear. Two or three will become satiated when eating a single ordinary sword.
Wandering the mine may prove to be hard on their armor and weapons, but the creatures attack the characters only in self defense. Otherwise, they get out of the way or ignore them. Deeper in the creatures become gradually larger, until they begin to look like ten-foot-tall pot-bellied stoves.
There are several large chambers, but only the deepest of them have anything in them. One is a (cold) working smelter which has nothing metal within it. It is stone, and stocked with coal, coke, and charcoal. It appears to be able to contain about a hundred cubic feet of material, but there are no signs of gold, iron, or other common metals. There is black and red slag, mostly backfilling smaller mine tunnels, and a black metal crusted with white dust on the outlet of the massive crucible. (The tri-laterally symmetrical creatures ignore this metal.)
In the most recently worked tunnels Mik and a cadre of the black, stove-like creatures, (xorn,) are working at digging into newer tunnels. Mik's reaction will largely depend upon how the party has dealt with the xorn. Tolerance for their natural behavior will make him more favorable towards them, while wholesale slaughter will earn his hostility.
Mik can be cheerful and talkative if he likes the party, but malicious and cunning if he despises them. Ten of the largest xorn are his friends. How he communicates with them is a mystery, but they will obey his commands and work as a team against the PCs if hostilities erupt. If, instead, the party earns his favor, each will be gifted with masterwork adamantine gear to offset what they lost to the baby xorn.
Such gear will be unenchanted, but ready to receive such dwoemers as the owner desires and can afford. Also, as artifacts crafted by the legendary Mik, they will sell for double the listed price.
Finally, Mik will not appreciate his current location and occupation being revealed to the public.
DM note: the xorn cannot digest adamantine so the trace amounts in the local stone is concentrated in their waste. Mik has been collecting the waste to smelt it, reducing in stages from a ton of ore to about two pounds of raw adamantine. Forging produces another 50% waste. Mik is currently searching for richer lodes to increase the yields.
Feel free to add your own dungeon drops. The basic idea is for a night's adventure that can happen anywhere. Terrain specific locations such as cities or lakeshores are acceptable, but most should be able to happen anywhere.
The Rope Factory
In the labor quarter of town where large multi-family dwellings are cramed together and industries carve out space within them is a rope factory which employs about 100 laborers and another twenty or so specialists and professionals. A stream of carts and wagons bring supplies in through the maze of streets and carry away spools of ropes from twine up to the massive hawser cables used by large ships. There are no retail sales here; only bulk orders of rope to commodities merchants.
On any given day there will be close to 500 people working in the area directly or indirectly for the rope manufacturer. Six bars and diners and a rather run-down inn occupy the area, and a weaver of various sized coarse sacks for bulk goods uses the rope factory's supply chain. The area around the factory is more prosperous than much of the labor quarter, (though still quite poor.) The local residents are proud of and protective of their local enterprise, and jobs in the area are highly sought after.
At night, the area is controlled by the local thieves' guild. 30 to 50 young toughs 'protect' the locals and make examples of folk who don't belong there. They also supplement the local markets and pawn shops with upscale goods stolen from other parts of town.
The owner of the rope factory is one of the city leaders, but the factory and thieves' guild is run by Master Ropewalker Giles Fathrander. His public persona is of a jovial, but stern when necessary, benevolent uncle who is concerned with his family's welfare. Within the guild he is known for sudden, intense rages when subordinates fail him.
He is protected by a group of eight fighter/thieves and a person believed to be an assassin. In addition to protecting their guild Master, they get assignments from time to time.
For adventure hooks, consider the following:
A noble's retainer approaches the party thief asking for assistance in retrieving a seal, signet ring or brooch, or other item that identifies the bearer as a noble.
A local thief steals a valuable item from the party, and the locals suggest they might find it at a guild pawn-shop.
A fight between guilds threatens the stability of the city and the party is contacted as a neutral party to negotiate an accommodations between the rivals.