Bonus objectives, all of them
Then chain quests

And when you find them, there are spots to farm with literally non stop spawns that you can just pull them all without even moving (this is the fastest)

97 to 100 takes 3 hours or so, depending on your class, farming in nagrand at ironfist harbor.

I just quested, finished each areas completely, like getting the story achievement for the area, and did the bonus objectives (these are where it's at). Shadowmoon-Gorgrond-Talador-Spire and I dinged 100 by doing like 4 quests in nagrand. /played was 18 hours. I boosted a character to 90 then starting leveling so 18 hours 90-100. Also when I leveled in spires of arak I chose the thing where I get 20% exp while leveling in spires

this basicly !

could somebody tell me what "rares and treasures" are and where to find them

Handynotes and HandynotesDraenor

Rare=Rare mob, treasure=Box or something like that which contains some loot. Can be found anywhere in draenor.

No matter what point anyone wants their garrison at... i think that using potions in gorgrond is super important.

where do you buy 20%xp potions?

Innkeeper in your garrison!