Currently in legion, there is a bugged box in val'sharah where looting it will give you an item you can turn in for +artifact power. Simply put your interface options to auto loot and right click that box until you get bored!
Here is the location, it is inside a house in a tree, the tree is often bugged and you will just see a window floating in mid air, but the entrance to the house is fine.
You can get roughly 2000 artifact power in ~45 minutes, which while currently capped on spend, you can still keep looting the items (for when they unlock it) or spend it and have it added to your pool.
Enjoy.
Here is the location, it is inside a house in a tree, the tree is often bugged and you will just see a window floating in mid air, but the entrance to the house is fine.
You can get roughly 2000 artifact power in ~45 minutes, which while currently capped on spend, you can still keep looting the items (for when they unlock it) or spend it and have it added to your pool.
Enjoy.