Some good suggestions above.

I'd add:

Take a province you don't really want or need, sack it, strip it, then give it to the Holy Father.

I often do this at the time of the first crusade. I'll take Gaza and/or Acre and keep them but give Jerusalem to the Holy Father shortly after its conquest. Eventually, the Mongols take it from the Holy Father, die in droves on my stake-reinforced gates at Acre and Gaza, and then I take Jerusalem for myself.

Nothing makes the Pope happier than having a more powerful and better-landed faction -- just be sure that it isn't land YOU need for your empire unless someone like the Mongols will come along to give you another chance at it.