Level up a new job

[Whiteroom]

Queue for Dungeons.
Do the Low Level Roulette.
Do each Guildhest at least once, also do the Guildhest Roulette, may not seem like much but a Guildhest is only a couple of minutes long anyway.
Do FATEs.
Do what Challenge Log you can.
Do Levequests.

Dungeons are best IMO. Especially if you can score a friend to Queue with who is a Tank or Healer (Or both if you have enough friends)

This is pretty much it. Run fates in between dungeons as well. I like to keep it fresh, alternating doing fates one day and leves the next. If its heals or tanks, and you are 41 or higher, go for SV or better. Use food and fc exp buffs. If you still have some 2.55 or less main story to do, they will be giving out double exp in the new expansion.