In My Humble Opinion adds the ability to see what your current character links of a character you're looking at - just like in CK2 which is great for some quick RP
Prisoner's of War is a great small additional in which characters taken captive in battle don't just teleport to the war leaders dungeon and are instead held by the capturing army until the make it back to their borders.
I originally bought ck3 solely for elder kings 2. I prefer the original game over it now, but it'll always hold a special place to me for pulling me in, lol