Dungeons and Dragons. That final frontier of "well, yes, I'm a nerd, but I'm not that kind of nerd." So endlessly uncool. Which is a shame, because it's so much fun. And, for writers, it's a totally overlooked way to practice writing, make creative friends, and have a really great time in the process.
Before I played, I always imagined D&D as a stat-based fighting game, which interested me not even a little bit. That's basically the most boring part of video games for me. But although combat is part of D&D, it's actually a collaborative storytelling game. The best way to describe it is like an open-world, decision-based RPG computer game (like Dragon Age or Skyrim), except you can actually, genuinely, do anything you want. You can approach problems however you please. You can flee from the city under attack to save your own skin. You can stab the king in the middle of him giving you a mission. You can do anything, as long as you accept that your actions will have real consequences too.
In D&D, you control one character -- but only one. You develop them and their relationships over weeks and months and years, and unlike writing fiction, you don't get to decide what happens to them, only how they react to what happens, and what they'll attempt to do next. Then it's down to the other players to figure out how their characters would react, the game runner (Dungeon Master or DM) to decide how non-player characters react, and down to the dice to decide how successful your attempted actions might be.
Not that the dice run a dictatorship on the story, like I once imagined. You get boosts or penalties for your attempts based on your character's stats (a charismatic character is going to have an easier time deceiving people than a socially-awkward one), on how good your strategy is (you'll have a better chance sneaking into a place through the back door at night after making sure all the guards are drunk than if you stride in through the front door at noon), and perhaps on how convincingly you roleplay the situation. It forces you to strategize, think outside the box, and dig deep into your character.
But you don't get to choose exactly what happens or what path your character will take. All you can decide is their personality and goals and play it out from there. Which makes it an awesome exercise in character development.
This was proven to me recently, when I started playing a selfish and ambitious human sorcerer called Kethra. In my head, she was going to end up on an adventure and learn to be a better person. If I'd written her story as a book, it would definitely have gone in that direction. Instead, she started a steady fall into Pure Evil, and was a hair's breadth away from betraying her entire party by the end. (Yup, D&D friends. If I hadn't been out of spells and low on health when you guys destroyed those black dragon eggs, I would so have fought you to keep one). It wasn't what I expected her story to be, but the way people reacted to her and the way her actions played out just kept leading her deeper and deeper into Team I Love Evil Dragons.
No one knows where a game of D&D is going to go, not even the DM. Things get weird. Sometimes you adopt little goblins called Droop and end up rolling deception checks to convince him that actually you do like his terrible cooking, really (and feel really guilty when you fail). Sometimes an attempt to gather information undercover goes awry because your friend wrote "Wizardy Bloke" in her notes instead of the relevant character's name, and you end up being blackmailed by a smuggler with some really second-rate goods for sale. Sometimes you make deals with chromatic dragons, because everyone knows that always ends well. And sometimes your DM wants to kill you when you keep trying to find out things about an ancient civilization that she has barely any notes about because it was just supposed to be a little side worldbuilding thing, seriously, stop doing investigation checks, there's nothing else to know, why do you hate me?
And it's great for learning how to stay true to your character and develop character-driven story arcs, even in super-plotty fantasy situations. It's definitely made me think differently about narrative and about how characters pursue (and are thwarted on the way to) their goals. And, of course, it's just plain fun. At least, it is if you like stories, really weird improv, and having great adventures with your friends.