My personal favorite exploit is giving the richest direct bishop/mayor a barony then plotting to kill him and recieving all that money cause he has no heir as a baron. I actually have it down to a science, I will kill off the first secondary castle I conquer till I own it, then give it to all my bishops and mayors till I get enough money to build a new castle in my capital duchy then rinse and repeat. Then once my entire realm is full I upgrade them all. All inside an ironman game. Oh and they even help you do the exploit by making every AI HATE a person with two types of government so everyone including rivals join you to kill them.