At level 5 you can probably solo the Waterworks quests, especially on lower difficulties. Go through them and make sure to hit every Mushroom node in the quests. You'll get tons of Sweet Whitecaps that sell for ~125 plat each and even more Strings of Prayer Beads that sell for ~100 plat each, both dependent on market price, of course. You'll also get quite a few Small Wooden Idols and Deadly Feverblanches that are about the same in terms of monetary value, but rarer. Some very rare finds include Amber Vials (~1k plat), Vials of Pure Water (~2.5k plat), and Tome: Prophecies of Khyber (~5k plat). There's also the possibility of finding a Small Eberron Dragonshard Fragment, which is easily worth 20k plat and can go for 25k plat. On the other hand, Icons of Khyber and Pale Creepers, which you'll probably find more than a few of, are all but worthless and should just be redeemed and the loot sold (Icons of Khyber *can* get you a +1 minor reptilian bane melee weapon, which is worth a few hundred plat on the AH, and Pale Creepers give you crappy armor that you sell for a few plat).
If you can't solo the Waterworks quests, then do Durk's Got a Secret. In addition to the collectables, there's a valuable rare spawn that drops a rare club called Muckbane (+1 minor ooze bane club). This weapon is extremely valuable because it's the only weapon early-game that doesn't take extra damage from smacking oozes. Bringing the Light is another option because it's much shorter than Durk's but offers more opportunities to get Vials (but significantly fewer for mushrooms and beads).
Finally, there really isn't any "tank" role in this game. Having 22 AC is really about the minimum you should have at your level to solo quests.