The Legion Invasions were a really good method to level from 100 to 110. And now that Battle for Azeroth started, they can also help us a little bit on our way to reach level 120.
Right now, level 110 characters still get 65.000+ XP per World Quest. 6 World Quests are available per invasion and they will take you around 10-15 minutes. So you just need 2 Invasions to reach level 111! Sadly, once your character reached 111 you only get 10% of the previous XP (from 110 to 111) for finishing the quest.
You can find more information on invasions at https://wow.gameinfo.io/invasions
By the way: You can take a ride on a Darkmoon Faire carousel (currently available) to get a 10% XP-Buff that lasts 60 minutes.