Three things from my current game, 1) Western Europe was split, primarily, into two kingdoms, France and Bavaria. Bavaria was larger and their kings remained stubbornly Catholic, combine this with their holy orders and the fact that I'd rather deal with Muslims, Mongols and Aztecs and they were a disproportionate threat to my Orthodox brethren. I noticed an opportunity when the third kingdom in WE, Lotharingia awkwardly wedged between the two, had a female ruler, an elective monarchy, and a son (of a different dynasty) ruling France. There were times when her vassals seemingly wanted her kingly son to rule but she'd keep switching it to a dynasty member. So I killed them. And killed them. And killed them some more until she eventually just gave up and her son remained the elector. Queen Helvis "The Holy" died of depression. She was the last independent Capet ruler. I'm not actually sure I consider this one 'fun.'
2) My daughter was abducted by some horde mofo. I tried being subtle about it at first, taking the intrigue focus and attempting to free her that way but while we made contact with some guy it never went further and in the meanwhile my daughter was having to breed horse lords. So I did what any concerned parent would do and systematically killed the entire family, including the son she had borne from her abductor. They were the Mordvanids.
3) My BFF, Cleopatra the Alchemist, was killed by the Hashashin. I couldn't immediately get to Alamut so I settled for killing successive Grandmasters, over a dozen (assassinating most, killed two in a duel) before I got the county and razed their little hideout. They came back two centuries later, apparently still in Alamut but they haven't killed anyone of mine so I've been tolerating them.
Edit:
Also I got the pirate stealing the recipe for Greek fire so I had him excommunicated, blinded him, castrated him, tortured him three different times, humiliated him, read him bad poetry and then made him the Court Jester.