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.
VIET and also RICE are great for this. I think VIET is mainly/mostly general flavor events while RICE adds stuff to specific regions and includes decisions. In combination they can really make the gameplay more varied from a vanilla run.
Although when I checked earlier this week one of both of them hadn't be updated for 1.9.1