Players can start their reputation with Cenarion Expedition by heading to Zangarmarsh in The Outlands. It is recommended that you start your honor grind by doing the instances, killing the Zangarmarsh mobs listed below, and turning in 10 Unidentified Plant Parts to Lauranna Thar'well in the Cenarion Refuge.

Reputation rewarded for killing mobs will no longer automatically deprecate. This means trivial mobs will continue to give out their full amounts of reputation on kill for the majority of cases in the game (level 70 creatures in Stratholme, for example, will continue to award the full amount of rep to level 80 players seeking to boost their Argent Dawn faction). 

Cenarion Expedition Instance Reputation

The Slave Pens and Underbog instances will give reputation through friendly on normal, Steamvault will give reputation through exalted. All instances will give reputation through exalted if they are done on heroic difficulty. All of the instance reputation totals are based on the amount of trash mobs you clear, the more you kill obviously the more reputation you will accumulate.

1. Slave Pens will grant 650+ reputation if done on normal difficulty and 1,800+ reputation if done on heroic difficulty. On normal trash mobs will reward 5 reputation and bosses will reward 70 reputation.
2. Steamvault will grant 1,600+ reputation if done on normal difficulty and around 2,300 reputation if done on heroic difficulty. On normal elite trash mobs will reward 12 reputation, non-elite trash mobs will reward 2 reputation, and bosses will reward 120 reputation.
3. Underbog will grant 1,000+ reputation if done on normal difficulty and around 2,600 reputation if done on heroic difficulty. On normal trash mobs will reward 15 reputation and bosses will reward 250 reputation.

Cenarion Expedition Repeatable Quests

1. Can't Get Ear-nough is a repeatable quest in Borean Tundra. You can obtain this quest from Arch Druid Lathorius at the D.E.H.T.A. camp, and it quest requires you to obtain 15 Nesingwary Lackey Ears (they drop from various humanoids in the Borean Tundra) rewarding you 150 reputation.
2. Coilfang Armaments drop off of mobs in the Steamvault, Heroic Slave Pens, and Serpentshrine Cavern. To be able to turn the armaments in you must first obtain Orders from Lady Vashj which drops in the Steamvault. Once you turn in the orders you can now turn in armaments to Ysiel Windsinger located in the inn of Cenarion Refuge, rewarding 75 reputation.
3. Unidentified Plant Parts drop off of the mobs in Zangarmarsh. 10 of these can be turned into Lauranna Thar'well in the Cenarion Refuge rewarding 250 reputation through friendly.
4. Uncatalogued Species has a low chance to be obtained from the Package of Identified Plants you receive after turning Unidentified Plant Parts. If you receive an uncatalogued species return them to Lauranna Thar'well in the Cenarion Refuge rewarding 500 reputation through honored.

Zangarmarsh Mobs

1. Bloodscale Overseers reward 5 reputation until friendly.
2. Bloodscale Wavecallers reward 5 reputation until friendly.
3. Darkcrest Slavers reward 5 reputation until friendly.
4. Darkcrest Sorceresses reward 5 reputation until friendly.
5. Dreghood Drudges reward 2.5 reputation through friendly.
6. Steam Pump Overseers reward 1 reputation until friendly.
7. Terrorclaw rewards 7 reputation through honored.
8. Wrekt Slaves reward 2.5 reputation until friendly.

Cenarion Expedition One Time Reputation Quests

1. Cenarion Expedition Quests (WoWWiki.com) / (WoWHead.com)