Means of getting reputation
Repeatable Quests ("turn-ins")
Plants of Zangarmarsh.
"Bring 10 Unidentified Plant Parts to Lauranna Thar'well at the Cenarion Refuge in Zangarmarsh."
You can turn these in until you reach Honored reputation. Yields 250 reputation per turn-in. You need 360 Unidentified Plant Parts to go all the way from Neutral to Honored. They are not BoP, so you can sell/buy them on the Auction House.
Bring the Uncatalogued Species to Lauranna Thar'well at the Cenarion Refuge in Zangarmarsh.
When you turn in Unidentified Plant Parts, you will be rewarded with a Package of Identified Plants. These has a rather low chance to contain an Uncatalogued Species. These can be turned in for 500 reputation, all the way to Exalted. It's recommended to save these until you hit honored and can no longer get reputation from turning in the Unidentified Plants.
Preparing for War
Obtain Coilfang Armaments from naga at Coilfang Reservoir and return them to Ysiel Windsinger at the Cenarion Refuge in Zangarmarsh.
These can be turned in all the way to Exalted. They drop from creatures in Steamvaults, Heroic Slave Pens and Serpentshrine Cavern. Yields 200 reputation the first time you hand in one, 75 reputation for the subsequent hand-ins. It's recommended to save these until you hit honored and can no longer get reputation from turning in the Unidentified Plants. They are not BoP, so you can sell/buy them on the Auction House.
- Nagas in Zangarmarsh yields reputation until you hit Friendly with Cenarion Expedition. 1-5 rep per mob.
- Terrorclaw yields reputation until you hit Revered. 7 reputation per kill.
- Hungarfen and Swamp Lord Giants in Underbog yields reputation until Revered.
- Steamvault monsters yields reputation all the way to Exalted. A full clear equals about 1600 reputation.
- Slave Pens gives rep until Honored.
- Heroic Slave Pens gives rep until Exalted.
- Underbog gives rep until Honored.
- Heroic Underbog gives rep until Exalted.
For a list of all available quests which yields Cenarion Expedition reputation, please click -here-.
Neutral to Honored
Hand in 360 Unidentified Plant Parts, which should get you to Honored Reputation. These drops from the various mobs in Zangarmarsh, but can usually be bought relatively cheap on the Auction House as well. I advice you to buy out 360 plant parts immediately when you get to Outlands (level 60), so that you can start doing the quests at the appropriate level (starting at level 61). If you wait until level 70 with doing these quests, some of the reputation might be lost.
Honored to Exalted
Do the Cenarion Expedition quests, hand in the Uncatalogued Species you got from turning in the Unidentified Plant Parts earlier. Run Steamvault for reputation and hand in any Coilfang Armaments you find. You can also buy Coilfang Armaments on the Auction House if you wish. I can't recommend running heroics for the sole reason of getting reputation, as you get about the same amount from a normal Steamvaults run, which is much quicker.