Questing!
With the help of Vot & Wiz, we went to Onzozo to hunt for a key. We "skill-up" for about an hour with no keys drop. We had decided to stop at 2a.m. and just 5 mins before 2a.m, the key finally dropped!
As Peach does not want to make another trip to hunt the Treasure Chest, Peach stayed in Onzozo to hunt for it. It was a lucky night as just a few steps down the tunnel, Peach spotted the Treasure Chest! After waiting patiently for the Scorpion and gobs to turn away, Peach finally got the song sheet (key item)!
Peach made a trip to Castle Zvahl the next day to hunt for the Treasure Chest (got a key during a coffer key hunting trip). This time it wasn't as lucky. Peach ran through all the spots and the Treasure Chest is no where to be found. Finally, Peach decided to cast Escape and run through all the spots again. It was a good decision made, as the Treasure Chest is found near where the Dark Spark NM is. Hurray!
Went back to Port Bastok to turn in the 3 song sheets and got a reward of 10,000 gil. Now Peach can start another quest which earn her the Siren Flute (That's what Peach is missing for her Bard job)!
Thanks Vot, Wiz, Felix, Darkx, for helping me with the keys in Sea Serpent & Onzozo!