Hey guys. I'm coming to the end of my current campaign with my party, and as it's the Christmas season soon (and one of my players is disappearing till after the new year), I'm going to do a collection of oneshots. Does anyone know of any Christmas-y themed oneshots that I could run on the session closest to Christmas? Alternatively, could anyone suggest any good guides to throwing together a oneshot, and I'll happily design my own. 

I’d suggest checking out the DMs guild for the Christmas-themed oneshots.

first thoughts are Hogfater by terry pratchett, i've made my own designed around that for a christmas one

i usually inflict some form of hogmanay themed one on my players too

That one usually includes a desolate moorland with a castle on it  wild haggis i.e. extremely vicious rabbits or something similar, a banshee except that it's a set of bagpipes with the stats of a banshee. and a cauldron of porridge  which has to be defeated to escape back to the real world.
this has Ac 20 or so and lashes out with grappling tendrils of sticky porridge which burn their target, the quest might include feegles and should also include some ridiculously bad puns.
such as another past monster which was a ball of carrot coloured curly fur with a body inside it about the size of a tribble which insulted people in a very bad glaswegian accent and attacked by springing into the air and delivering a devestating headbut. the pun was in it't's name being a Nnoyed (that one got dice thrown at me)
it's usually a good starting point in having the party get sucked into some mysterious swirly thing while out travelling and deposited on the dark mysterious moorland,
the more ridiculous you can make things the better.

I cobbled together a Christmas one-shot out of the Aquisitions Inc Penny Arcade comic where the team is "hired" to go deep into the Feywild's winter realm to steal the Maledictum Scriptorum or "Naughty List", an ancient artifact tome that records the bad deeds of every living being and auto-sells a person's soul to the nine hells when they have accumulated enough karmic debt. Anyway, all I did was find a nice fortress map, and sprinkle in some themed monsters. If I remember correctly, I used a two-headed Ettin for Santa as the feylord of the realm, deep gnomes for his 'elves', antlered centaur guards as 'reindeer' stand-ins, a couple gelatinous cubes, an ice-themed elemental or golem to guard the book. Somewhere in the fortress I tossed in a dryad helper NPC that was bound to a pine tree with mystic chains of garland that the party could rescue.

Why the party needs to steal/destroy the book is up to you. In the comic, the party wizard somehow does enough bad things to get put in the book and hires the party under illusionary disguise to steal it so he doesn't have to pay up with his soul. In my game, a Teifling NPC that the party is fond of gets a notification Christmas card that her name has been added to the book for some of her darker past deeds that she is trying to attone for and Santa will be coming to collect her Christmas Eve to deliver her to her new masters. 

I’m writing a heavily homebrewed one to run for my family that I’m also building on Roll20 to test run(since I’ve never DMed before, practice seems smart).  The party is hired by community of 1/2 elves (the other 1/2 is gnome, so they look like Christmas elves) to rid them of a druid who is trying to overtake their magically built, ever-wintery village for a beast sanctuary. It’s a low level adventure full of fun magic items and themed monsters, and easter eggs of cultural Christmas fun(like there’s a squirrel that jumps out of a Christmas tree at the party like in Christmas Vacation). The adventure starts as Santa scurries back up the chimney, so the party has their presents and gets to use them while on the adventure. But they’re either consumables or mostly goofy items that even if they decided to keep them for the future would hardly make them OP.

I homebrewed stuff specifically that their character would find fun to keep. Like I saw a silly magic item online called the Blanket of Fortitude that I tailored for my daughter(basically a blanket that magically becomes a blanket fort. Functionally, a tent). All 4 of my kids are being given colored strips of cloth with eye holes cut out(ninja turtle masks) that gives them proficiency with the weapon associated with their color and allows them to use their STR/DEX modifier with their off-hand attack with that weapon while wearing the mask. They’re all being given Ugly Christmas sweaters that make you feel festive, so ADV on CON saves, but you look like a dork, so DisADV on CHA based skill checks while wearing it. That’s just a sample. If I don’t see a Christmas fun magic item post already made here, I’ll make a separate post with all my ideas so others who want a lighthearted fun list of items to play with can start working them in.

But the main idea is it’s easily survivable, slightly silly, but in the world of Faerun somehow plausible, and generous with gifts/treasure, just now the kind of +2 armor people want to build their tier 3 characters on.