You know what never gets old? Making the Pope and with him the rest of the entire Catholic world your bitch.
It's great because according to Catholicism, the Pope answers directly to God. As my vassal, the Pope answers to me. Therefore, according to Catholics the world over, I have become God. After all, I get to tell him who to excommunicate, who to call crusades against, hell, even whose titles rightfully belong to me. It's absolutely fantastic.
I have to say, when you can make it work for you, religion suddenly doesn't seem quite so shitty.