I managed making a songstress outfit in one and a half days from old stuff, tho maybe more work than what you want, so an easy cosplay can be put together from unusual stuff you already have- if you happen to have lotsa sparkly things, some flowy fabric and a few basic pieces of clothing you can make up your own interpretation of an amano FF character? they're pretty open to interpretation most of the time, so that's one to have fun with... Amano's princess Sara, Celes, Lenne are all easy if you have either a dress or top similar to one of them...I say that cos A friend happened to have a dress similar to Amano Sara's, so some pretty bracelets in her hair for a crown, more on her arms and a necklace sealed the deal. therwise Reinna is right- Selphie and Rinoa are easy, Tifa too if you have the skirt, top and suspenders, as well as any Turks from FFVII if you have a suit :P
BTW since you're mentioning Moogles and Chocobo's, don't forget Tonberries! A brown hoodie cloak thing, greenmask/green face paint with big yellow eyes
and lantern would look cute too... so yeah, hunt for what you already have at hand first
enjoy the concert!