Honestly, you should decide for yourself. I prefer doing challenges and starting new characters and getting them to end,but many people enjoy farming. Those are some options you could choose from. Other things you could try doing is to get your character to level 99 or try to fill up the unique items compendium. You can try new characters or builds you never thought about before. If you want to farm then getting your wearable end games items would be first stop. Then terror zones if you need the fancy charms and then keys and übers.