Repetitive daily quests are a legitimate method of generating significant cashflow. Players who spend several hours a day doing daily quests can often generate 100 per hour. Many players have funded the purchase of their elite flying mounts solely through doing daily quests. Another trick for higher level players is to do level 70 dailies as a level 75-77. If you have a hankering to get a Netherdrake, for instance, and you already have a fast flying mount (which is a pre-requisite for the drake), you will find your mid-70s an ideal time to go get that drake. Not only will the quests be a lot easier to do at level 75+, but the dailies will pay about 1200 as you level rep, and you'll get XP along the way to boot (albeit not as much as quests in Northrend.) So if you're looking for a break from the grind of leveling to 80, and want to kick back, spank some level 69s and make some decent money along the way, doing lower level dailies can be a fun way to make some extra cash.
Swim Speed Potions (used especially at lvl 37 for Faldir's Cove in Arathi Highlands and lvl 57 in WPL). Try to get multiple stacks of these. I also recommend keeping a stack of these always in your bags. You will frequently pass rivers and lakes and can use these potions to pass them faster through out the guide. Please note that these potions do not stack with Druid's Aquatic Form and Glyph of Aquatic Form.
Elemental items (Primals, Eternals, etc.) can be a good source of money, because they are always in demand on the AH. See which mobs you will most benefit from farming, then set out for a few hours. In the process you will also most likely collect significant vendor trash, and may get other profitable item drops as well. Note, however, that older elemental drops usually lose profitability compared to newer ones, i.e. Primal Water sold well to level 70 players in BC, but sells far less well to level 80 players in WotLK, because level 80 gear requires Eternal this'n'that.
AQ20 is a really nice transmog farm. The worms called Silicate Feeder are acutally the mobs with the best loot in the raids so be sure to loot them. They are found between the 3rd and 4th boss. Be sure not to kill ANY of the mobs in the room with the 2nd boss as that will start the encounter for the boss. You can go through the whole instance without killing any bosses, so you can choose to run the entire raid or exit after killing the worms by creating a group in the group finder and leaving it.
Just before Battle for Azeroth goes live, you will want to log all of your characters out in the zone of Silithus so that you can acquire the quest to talk to Magni and obtain your Heart of Azeroth (refer to our Heart of Azeroth guide for more information about this item). This is the new artifact-type of piece of equipment that you will use for all of the Battle for Azeroth expansion. Once you acquire your permanent neck slot piece of equipment, get back to Stormwind or Orgrimmar to begin your scenario that leads into the introductory areas of Kul Tiras and Zandalar.