My guess is that when you were a kid, right, you were one of two types. You either used your action figures to play out imaginative (if potentially weird) stories, or you picked one of them up and went "I bet I can make Lion-O's legs go in the splits so far that they fall off", before proceeding to test your hypothesis.
Players of The Sims can broadly be divided into these two categories as well. And as far back as the first in the series, the latter set have been pushing the game to its tensile limits by introducing play challenges. In these, you create a Sim, or a family of Sims, and then play their lives out according to a set of very prescriptive and often torturous rules. Now, in the age of The Sims 4, these challenges have been refined. Sub-rules have been added. Players try to complete even the most difficult ones in a 24-hour span. It is compelling. And also frightening. Let me introduce you to some of my favourites.
The Rags To Riches Challenge AKA The Rosebud Challenge
I guess technically it's now called the Rosebud Challenge (named after the money cheat), but I'd bet most people still know this one as Rags To Riches. It was popularised by a bunch of Sims YouTubers doing let's plays, but probably the best example is this one by James Turner, who's one of my favourite speed builders:
Rags To Riches games are about starting with nothing, and working your way up to being a millionaire with a house. You start on a totally empty lot and have to get food and money by fishing, busking, and so on. You can't apply for a job until you've made §500, and for additional difficulty, you can agree to only buy things to improve your lot once a week - after your bills come out of your account. It is, essentially, a jaunty and sanitised simulation of homelessness, and on that basis should probably give you pause for thought.
But it is also, when viewed dispassionately and just as a set of rules, a perfectly crafted challenge. It has clear goals, is difficult but achievable, and will let you see continual progress. Listening to someone say "how does it take Sims to die of hunger? It's a long time. I think I've tried to kill Sims from hunger before," carries its own kind of macabre fascination, as well.
The Apocalypse Challenge
I've yet to find a video that really showcases a good run at the Apocalypse Challenge, but I really love the concept - specifically because you have to spend time building a loving family and raising a child as part of it. Then, as soon as they reach the young adult age bracket, you assume the nuclear apocalypse happens, kill their entire family, and move the designated survivor to a totally empty neighbourhood. You build a shack for them in an empty lot, which has to have a raised foundation or be on stilts because, quote, "the ground is still covered with radioactive dust".
The Apocalypse comes with an exhaustive list of restrictions, organised into chunks, and you can only remove one set of them once a Sim reaches the highest level in one of the career paths. Seriously, there are so many rules. But it'd be interesting to see someone play this, especially with the new off the grid stuff, and things like bug farms, coming in the Eco Lifestyle pack.
The Decades Challenge
There are elements of this challenge that I find genuinely hilarious. The idea is that your Sims kind of live through history (every time a generation of child ages up into young adults, or teens if you prefer, a decade passes), starting in 1890. There are rules about things that make sense, like when you can introduce running water into your home, but if you really want to, you can also optionally gamify historical misogyny, racism and homophobia. Fun!
The actually funny bit, though, is that eventually you pass through huge, world shaking events, and the rules do their best to model their effects in the game. So, for example, any Sim coming of age during the 1930s has to take either the Gloomy, Noncommital or Mean traits, because they lived through the Great Depression. In decades of war, Sims falling into the conscription bracket have to be fed to the carnivorous Cowplant on the roll of a dice, to simulate dying at the front. The sentence "first two children (Of either gender) are sent to war (cowplant) as conscripts or volunteer nurses" is just objectively very funny.
The Black Widow Challenge
This one is almost pathological, I'm not going to lie. You create a young adult Sim, give them the traits Materialistic, Snob, and Romantic, and proceed to romance and marry a neighbourhood Sim. Then, meet a new love interest, move them into your house, and make sure your current partner sees you cheating on them.
Then kill them.
Marry your love interest and move into their house. Repeat the process 10 times, taking the urns/graves with you as you go.
The 100 Baby Challenge
I would characterise the 100 Baby Challenge as actual body horror. You create a young adult sim to be your matriarch, and then force her to have 100 babies, at peak efficiency, by 100 different Sims (referred to in the rules as "donors"). I imagine some kind of spreadsheet will be necessary. You have to have normal aging on as well, so this woman has to spend basically her entire life pregnant, whilst constantly flirting with the Sim identified as the next donor. The grind is demonstrated in this popular lets play by Kelsey on Buzzfeed Multiplayer, which has been going for almost two years at this point.
(Kelsey made the bold choice of making a matriarch who looked a bit like her.)
You can't hire a nanny, so must juggle your hyper fertility with childcare. Once your matriarch ages out of her childbearing years, the responsibility passes to her eldest daughter. This is basically human women as Ant Queens, and it is a holy terror.