I like to go to the Limsa lower docks or Outer La Noscea's Hermit's Hamlet. Both are really nice looking areas and the Hamlet is like a hotspring. If you both are good at role-playing then you could pretend you're at the Bismark for a nice dinner. There are lots of places that will suit whatever kind of scenery you are looking for. Gold Saucer may be an obvious choice for playing around at. La Noscea is great for dates as it has Costa Del Sol (don't think I need to point out how it is a beach), Wineport (wine tasting and the nearby jungles. But I guess it really depends on what you want to do; if this is the place you want to propose at then La Noscea, and some parts of the Shroud, may be just want you are looking for. GL.