Even if you don’t have flying in WoD be sure to get the WoD handy notes. If you have flying simply fly and pick up all the treasures that you can and do bonus objectives. Not every single piece of treasure will be worth it though as some are hidden in caves or are just hard to get. So if you find yourself spending more than a few minutes finding a treasure, just skip it and move on, there is plenty of treasure to go around.
If you've been away from WoW for some time, you may not know that the game's Legion expansion focused on powering up an impressive "Artifact Weapon" throughout the duration of the expansion. In short, players have been using the same weapon for about two years now. Artifact weapons were pretty cool and bosses did drop Relics which were used to upgrade them. But after two years of using the same weapon, many players are looking forward to the thrill of having an actual weapon drop off of a raid boss again.
These are a genuinely challenging and dynamic way to play. No two are alike, and the A.I. actually does give you some real opposition. Island expeditions are like dungeons, available in normal, heroic, and mythic difficulties. That means you can press into higher difficulties for more of a challenge. Of course, the PvP version pushes everything to the max.
Update: The reason for less experience from gathering is due to how Blizzard revamped professions and broke them down by expansion. In order to get full exp from gathering you'll simply need to find a profession trainer for the relevant expansion. (WoD you just mine or herb the first one ypu come across and the game gives you a scroll for learning WoD gathering).
Tiragarde Sound and Stormsong Valley are both fairly similar zones, but Stormsong is a little longer, meaning that you might not even have to finish it to hit 120. Tiragarde also comes with a bit of a warning. There are plenty of side quests to do in the zone after you finish the story quests. Don’t do them. It’s not worth the time or the experience you’ll get.