Anyone heard of the Immortal Dynasty challenge?
Look it up, it's weird.
I was inspired to make up my own Sims game challenges, so here goes!

Rollercoaster Weight Challenge: No weight gain cheating! Start a Teen Sim at lowest weight. On their Adult birthday, they should be near max weight. You have 1 week to get them to max weight. They then have to have a child that's at max weight until Teen, when they'll size down to min weight the same way the first Teen did with gaining. Then have a min weight baby that stays min weight, then a max weight baby that stays max weight. Make them all males, then have the last one have a female baby. The baby must grow up to outweigh their lover.

Supernatural Immortal Easy Challenge: Start off with a Human. Get them turned by a vampire or werewolf. Then, have a child with whichever one they weren't turned by. Make sure when they are old enough to have a baby with somebody, it MUST be another Supernatural they're having a baby with. If any baby isn't supernatural, make them a witch. They'll supply the supernatural potions. The other non supernatural babies drink the potions. The cycle must continue 8 generations and all 8 must be Supernaturals. All of them must live to see the eighth generation. Then, stop the aging.

Grim Reaper: Make a main Sim to be the Grim Reaper. They must kill off all the other Sims in the household all in different ways and have the ghosts show up and live in the house.

Try these, the Dynasty Challenge, or maybe even your own rules!