Go back to Smelter Demon to fight Pursuer again
Do all 3 giant memories, the Dragon memory, and kill the giants before the Rotten
Join the Darkness covenant, beat all 3 areas, fight and kill Darklurker
Kill Ancient Dragon
Kill Vendrick, get his soul + armor
Spend 50k + souls on McDuff to get his cheevo
Spend 12k on Ladder guy for his cheevo
Join every covenant for cheevos
Buy all the items off the covenant leaders
Buy all the spells from all the vendors
Revisit Wellager and the Vampire, they will give you items for talking to them again after killing the giant lord / visiting the king.
Invade/Kill Priest Lady
Kill every NPC in the game, except for the old item lady in Majula. Off the top of my head the ones worth killing are - McDuff, Vendor in Iron Keep, Armor salesman, Blue Sentinel covenant leader, Red Sentinel covenant leader, Pate/Creighton, Firekeepers, Hex mage salesman, and Pardoner priest. All of these drop something good/unique, or unlock an armor set on the item lady in Majula. The rest of the NPCs have some drops, but I recall them being mostly garbage, useless, or stuff you've already gotten before. I killed every NPC possible except for the headless guy, moonlight sword guy, and the masked lady (forgot these). Not sure what they drop when killed.
Go to item lady in Majula to buy a lot of the NPC armor sets of people you just killed, you're going to need 100-200k+ souls for this.
Optional, but might be worth doing now:
Use Bonfire Ascetic to get some of the NG+ boss souls if you want to use their weapon for NG+.
Try to do the Moonlight Sword guy or Mask lady quest line with bonfire ascetics if you didn't summon them for those bosses before. It's still possible to do this post game as long as the NPC is still there. However, the last spot the Mask Lady goes to is a 1 time conversation, once she leaves there youre SOL until your next playthrough. Also, since they have to live through these fights, the bonfire ascetic version of some of these bosses are super difficult to accomplish this on. NG+ version will be the same though.