I'd recommend decorating the cake with something that goes over a large array of Final Fantasy titles, but isn't recognized for their personality, but the simple fact that they're well known. For example, moogles, cactuar, and chocobos. But if you were to go with a character like Cloud, Cecil, or Squall, certain people may find certain characters dislikable, as they can be directly compared to other characters in the series that serve the same purpose. Also, using characters like moogles and cactuar make it so that no matter which FF a person has played, they've probably seen those characters somewhere.

As for the music, I think you should try to figure out which musical tracks she likes the best. There are some tracks that appear in multiple titles, but they aren't exactly the types of tracks that make you go "WOW!". They're more like "Oh, hey, that's this theme. Cool.".