Cyro repeatable quests,
PvP in general
Dungeons/trials
Dolmens
World bosses
Easiest way though is to have a training bar and swap stuff out while levelling. Once something is maxed switch it for something else and have the training bar active when you hand quests in.
Cyrodiil repeatable quests, switch to correct bar Before handing in quests. Any XP goes to abilities slotted on your Active bar. So even farming mobs in couldharbour will do the job pretty fast.
Add PvP loot drops for some risk/reward in this game.
Captain Morgan Society Zub
How'd ya feel like scraping the barncles off me rudder.. Matey..
The easiest way is to go to Couldharbour and grind zombies. Hit all zombies on spot and kill them using aoe spell.
There is faster way, but you need very strong character, or even two characters. Guards on gate to scrolls spawn every ~15s and they give a lot XP, you can level up your skills here with your friend. It should be faster than leveling in Coldharbour. You get 12k XP for 4 packs on gate, so 3k XP for one pack.