Weirdly enough, since I'm huge on planning out my sims' lives, the thing that makes me happiest is when something totally unexpected happens - beyond my control. Someone dies, someone gets pregnant by accident (with risky woohoo and ACR and all that), two people hit it off better than expected, etc.
Sometimes I tempt fate and don't pause the game when I get up to get a snack, run errands, or whatever. Some of my favorite games have been created that way. I had a house full of CSI sims (Grissom, Sara, Catherine, Warrick, Nick & Greg). . . all paired up and pregnant. Babies were had, I walked away, and everyone died in a fire except for Catherine & the babies. That was a REALLY fun game. Hectic, but so much fun.
Families without parents have happened too. . . they're a lot of fun. I like when my Sims actually have to work to make ends meet.
An affair with the mailman, teenage parents living on their own. . . I even had someone kidnap a toddler once.
Good times
