I update my long term plan for territorial acquisitions for myself and my allies including list of potential short term war targets, their allies, their rivals and the maximum manpower of their prospective alliances.
I watch the nearby AIs fighting wars, monitor their territorial gains and losses, and consider whether intervening would be beneficial to me.
I keep an eye on the Disputed Successions list to see if I want to fire off a quick royal marriage or claim throne somewhere.
I build buildings, shuffle merchants around, send out my colonists, upgrade my ships, rebuild my armies if I had to consolidate them in the last war.
Rearrange my diplomats, fabricate some claims for my next wars, look around for nations willing to be diplovassalized, check expire times on my claims and random cores that I want to release/feed.
Buy off all the prospective cardinals with the massive stockpile of papal influence I gained during the war.
Pray for any heir with more than one 3 in his stats.
This sounds like what I do, plus
-Improve Relations with friends, allies AND enemies, to try to keep the initiative in my hands
-Check the ledger extensively for information about my friends and enemies, to be able to make my short and medium term plans
-Do infrastructure work (building construction)