The way in which we are playing the game: http://adeptplay.com/actual-play/some-osr-sandbox-play .
Context: Player characters were chasing some bandits, but lost track of them close to the town of Saltmarsh (from a TSR adventure, U1 The sinister secret of Saltmarsh) and the fort of Morgansfort (from a Basic fantasy adventure of the same name at https://www.basicfantasy.org/downloads.html ). There is some dungeoncrawling in the Old island fortress dungeon near Morgansfort, some characters get to know the local illegal drinking place and the local baronet. One character has criminal connections and finds a weapon smuggler in Saltmarsh.
The Saltmarsh adventure assumes that characters march in for whatever reason, hear about a haunted mansion, investigate, find a smuggler hideout under it, defeat some smugglers, capture their ship, yada yada. It has two follow-up adventures and the series does include quite interesting challenges, which are nullified by the game master advice to have the players succeed and follow the script regardless.
The weapon smuggler? An influential merchant and also together with the smugglers under the haunted mansion. The adventure specifies nothing about this merchant or their purposes, so I determine they are in it not only for the money, but also because the smugglers have the merchant's daughter captured and taken; in truth the daughter is a pirate queen of sorts, but it is left ambivalent if how parents actually suspect this and willingly blind themselves to justify the smuggling, or if they are unknowing.
Anyway, one player decides that instead of going into a dungeon, they'll reveal this smuggler to the baronet and get rewarded for it. A good move in general; avoid the uncertain and dangerous dungeoneering and do some social engineering and intimidation instead, and maybe even do some good on the side. The baronet does offer a non-trivial sum of reward money (and thereby experience) for it. But the baronet does not want to go barging on the home of a wealthy merchant and member of town council without good grounds, so he musters his troops with an excuse and player characters are sent in first, so that if they find nothing, the baronet and the troops will simply go hunting for non-existing hostiles in the swamps as per the excuse. Plausible deniability for the baronet.
The plannin process was so that the baronet simply states that he can't just take a bunch of soldiers and march into the home of a council member; what are we going to do about it? And then the players brainstormed until their plan was sufficient for the baronet to accept, given the risks and rewards. As a referee my role was to tell what the players know about the present situation and what the baronet can tell them; had the baronet been more of a schemer, he might have actively suggested something, too.
Characters go to the smuggler-merchant's house, set up watch (one watchman almost catches a certain assassin from the haunted house; I add active NPCs to the local random encounter tables for exactly these kinds of reasons), break in later and the forcible interrogate the merchant, who under duress agrees to co-operate, given that they let the baronet know of this and promise to help her daughter. He lets the characters know the smugglers are somewhere under the mansion, yet he does not know any details.
Here we see the particulars of the rules chassis we are using (D&D 5); one of the characters is an aarakocra and can fly, while another is an eladrin and can teleport a bit. Flying around revelas the sea entrance in the smugglers' hideout. Three player characters go that way. They have enough rope to climb to within 20' of the sea surface (I first scan the adventure or decide how tall the cliff is and afterwards we check how much rope the charactes have). The wind is rising, but they decide to go in anyway; a surprise attack from rear, what could go wrong? The thing that goes wrong is that there are lots of enemies there and the characters have very hard time escaping, especially after they decide to burn the smugglers' boat so that the smugglers can't get away. Some combat later, the eladrin is the only one to escape by swimming against the wind and the teleporting to the rope, while the others are imprisoned and interrogated.
Meanwhile for the mansion itself; there is investigation, some treasures are found and others missed, some stirges are killed, the rotten floor collapses under someone, and so on. They discover one way down (stairs to a cellar next to the kitchen), but miss a hidden trapdoor that is the other entrance. The assassin is not met; the adventure assumes the assassin is found as a captive and taken along only to betray the characters later, but no such luck here as the smugglers did not have the time to set the assassin up in the manor. The baronet's force arrives. The strike force (couple of player characters and couple NPC knights and some soldiers descend into the cellar, avoid and burn some rot grubs, make noise, alert the smugglers and eventually find the secret door to their hideout.
The smuggler hideout has two viable exists (their boat gone), both from the same big room. They have prepared for confrontation and for escaping with some of their loot ready at the exit, too. The captives are there, knives at their throats, ready to be used for negotiation. Some knights are the first to charge in and quickly deal with the frontline of the smugglers (gnolls, I think). The player of the aarakocra tells that they will try to burst their bounds and shake of the smuggler holding them. Okay, versus test (probably athletics or even initiative; it has been a while); you win and you throw them off, rising to standing position; you lose and they slit your throat, so you are dying; and you do have disadvantage due to being bound and all and the smuggler is explicitly holding you there anyway. The dice are rolled (in the open, of course, since this is an important moment) and a well-loved player character is dying. Some death saves later they are dead.
Meanwhile the fight continues. The smugglers do not do well, so I check morale (a wisdom save; technically not a part of D&D 5, but our point is not so much to play D&D 5, and people being afraid of their lives is something that clearly adds to the game and the tactical and strategic depth). The smugglers start running, the other captive is thrown hard against the ground (the smugglers have not reason to kill them, after all; luckily it was not a gnoll holding them). When the player characters figure this out, there is a moment of confusion until some run back up through the kitchen and others follow the other entryway. The smuggler boss, an illusionist (I use their spells from AD&D; no reason for them to follow the same rules as player characters), manages to escape and is followed by one the eladrin, who much later finds out that a single higher level illusionist with thrown daggers and good dexterity is a deadly opponent to a first level fighter, but the later the illusionist comes to find out that being alone in the woods makes an urban snob easy prey for determined hunters.
Most of the smugglers are caught. There is lots of loot in the dungeon, so after some discussion word is sent to Saltmarsh that people will get paid if they haul out the loot; the assassin is one of the volunteers. While waiting for them, the player characters explore the underground. Treasure that belongs to the baronet, some green slime that surprisingly does not corrode one armour (it happened to fall exactly on the character wearing a full plate the discovered earlier which later turns out to be magical; very good luck there, it could easily have been deadly otherwise) and one dead-end room with skeletons. After killing the skeletons the players are really puzzled about it all; no necromancy in the adventure and then a dead-end room nailed shut with armed skeletons? They don't think of searching for secret doors, so the most interesting treasures of the adventure are missed. Such is life.
The assassin makes a final brave attempt to scare away the folk by capturing a random hireling, killing them, cutting their throat and throwing them down on the manor lobby. The player characters are too clever for this and start chasing for the assassin, who does manage to avoid them for a while with the home advantage. There are dry bushes on the outside of the manor and a missed fire spell hits them (missed attacks with any effect whatsoever is not part of D&D 5 rules as written, but we certainly take them into account when appropriate) and the characters choose to burn down the garden, inspired by this. Windy weather and uncontrolled flames, in spite of some noble attempts, finally devastate the manor. The assassin escapes.
Later: Some goblins move in in the abandoned caves (restocking the dungeon). The player characters have a choise of waiting substantial times for the baronet to summon their reward in gold, the now-burned plot of land where the manor stood, or the rights to an abandoned fortress with a productive mine in the nearby mountains; they choose the latter, which turns out to be full of orcs. The assassin kills the smuggler-merchants but does not get their hands on the smuggler leader, who is imprisoned. The smugglers are building a wall towards a swamp area infested with lizardfolk. What the pirates doing the smuggling, the lizardfolk or the sahuagin, part of the follow-up adventures to this, are about has not been seen in play or mostly determined yet, though some signs are in the air. The player characters are currently active elsewhere.
The bird-man, as the aarakocra was known, was an estalished and well-like character. Their death was earned. The eladrin was a new character and well-played; they simply did not know what they were up against and, even then, it was a close call, but such things happen.
The scenario that we played here came out from an interaction of two adventures, one a typical starting town plus some caves, the other a more intricate adventure with lots of potential. The player choises in approaching the entire situation were crucial to creating the scenario.
The ruleset, D&D 5, is mediocre. It is somewhat cumbersome but not awfully so and does give the tools to resolve most adventurous situations. The characters are brimming with boring magic, which I find aesthetically unpleasing, but it does not really affect how fun playing is. The most bothersome aspect is that it neuters the consequences of many actions. Magic coming from demons, Cthulhu, or dark gods has no price. Shooting into melee is trivial. Resting we have house ruled so that not everyone heals completely with a night of sleep. We pay more attention to encumbrance and carrying then the (ridiculous) default assumptions of a random man on the street carrying 70 kg of stuff around while marching 50 kilometers per day and fighting some skirmish combats, all without getting particularly tired. These house rules were not particularly relevant during these sessions.