If you have all the story missions saved at the end before finishing them on another character, then you can resume them at the end boss with the new character and get all the xp from completing each story mission on hard. Takes 4-5 hours to get to 20. Also all the bounties when they unlock as you level up. I haven't done this but read it in another thread.

Bounties and I would say PvP but sometimes it takes to long

Bounties and pvp