I added comparatively few of these.
Dimensional Havoc
Oldie but goodie. A portal will invade the Hamlet, and each quest will be a single 20+ round fight against waves of mobs, some of which are adapted from Koala's Creature Collection with permission. I really like the quirk rewards - you can only use so many trinkets and so much gold. Doing the quests will get you Empyreal one-per-roster quirks (Empyreal Resistance, Empyreal Cloak, Empyreal Vigor, Empyreal Cunning, Empyreal Alacrity, and Empyreal Aptitude - but it can be difficult to make them go to the right hero), as well as "orange" quirks which are tied to killing specific portal mobs and can be given to multiple heroes, but don't need to be locked. One of the quest reward trinkets also allows you to farm consumables which give actually useful positive quirks.
Levantine Invasion
Black Reliquary backport/teaser adding a Vvulf-like invasion event. Very fun assuming you notice the gimmick. The BR backport classes do well.
The Manor
I wanted something to pad out the Darkest Dungeon, since it didn't seem right that even with an enlarged roster I could only traumatize up to 16 heroes. (Darkest Plus is still on Skymods but was removed from Steam for being buggy and not meeting the standards of Courtyard Plus and Farmstead Plus.) The Manor has rough-looking art, but it does what I wanted.
Normally it must be installed at the start of a new save. But it's possible to use the online save editor to replace the Shuffling Horror quest with the first Manor quest, which is what I did. I only got the second Manor quest (kill the Archpriest) after doing the third Darkest Dungeon quest, which might or might not be intended. If you're using Trinkets Plus, you need the compatibility patch.
Ringmaster Boss Fight
Nice to have a single-player use of the Ringmaster concept. However, while the fight isn't difficult for the prepared (as are most DD bosses), it is
arbitrary, more so than vanilla bosses. The "pls no modded corpse clear" on the Steam page hints at this early. You will also notice when going into the dungeon (a single fight dungeon like the Shrieker), you can buy anything except Bandages - because the Ringmaster opens with a party-wide 1 pt/25 turn bleed and has bonuses vs Bleeding. Cure skills still work (though it is actually possible for a mod enemy to disable them). You will also need pure rank 4 burst or DoT without relying on move or corpse clear, or keep the rank 1 minion alive so that rank 4 can guard him; even the Prophet allowed you to destroy the pews hiding him in rank 4.
The Ringmaster's Prizebox add-on mod is the intended method of accessing mod classes' Ringmaster trinkets if they have them.
Single Player Butcher's Circus Lite
Lets you fight parties of four in the Butcher's Circus aesthetic. Doesn't count for achievements and the opponents are based on Chogex's Fallen Heroes, not the Butcher's Circus implementations. I find this a more interesting way of getting Ringmaster trinkets compared to buying them or fighting the Ringmaster boss.
But it's not compatible with Singleplayer Circus Trinkets (which makes them appear in the Nomad Wagon), it doesn't drop mod classes' Ringmaster trinkets, and Unified Circus Trinket Pool does not support it. Fighting in the single player Butcher's Circus also gives out quirks which only work in the Butcher's Circus dungeon and can't be replaced by other quirks, so even the new "positive" quirks are net negatives if you want to use your heroes anywhere else, which wastes Sanitarium time and money and is just stupid. I edited this out.
Talon Mother

A package deal with the Falconer class. Send out a party with someone who has the Marked by the Flock quirk and the Talon Brigands will show up as normal battles. They are distinguished from normal brigands by their practice of bird-based necromancy, and they will drop feathers as proof you've killed them. You need to fight them at least twice in a dungeon for the feathers to be worth more than a gold stack. You'll also want either a no-damage/low damage stun or guard bypass/break, because they sometimes have a Talon Damsel hostage who guards them and deals stress to you, but will give you a stress heal if left alive.
Send out a party where all four are Marked by the Flock and the Talon Mother will appear
(Youtube video). Ideally your team should not be too position-sensitive. Unless she summons a mob you really can't deal with, it's best to focus on her, because she can eat/magic/idk the corpses of her mobs. She also does Mark and bleed. Defeating her gives you two unique trinkets.
Wildlanders Backport
Compared to Wildlander Invasion, Wildlanders Backport is not as polished in presentation and the Wildlanders can show up in normal dungeons, which can be annoying if you're more prepared for the dungeon than for them. However, I recommend it over Wildlander Invasion because Wildlander Invasion uses the old, boring version of Warhawk Matron where she just sat in rank 4, while Wildlanders Backport has the version where she moves forward as she loads her different ammo types and will nuke you if she reaches rank 1
(Youtube video). Wildlanders Backport also has the Steelpath Matron.