If your wife uses the app, it will be hard to keep it a secret. Maybe leave little Hawaiian-inspired things around the house, in the suitcases, etc.
My favorite surprise when I was a kid was when my parents mailed a letter from “Mickey” telling us that he just needed us to come celebrate the millennium with him. They sent it about a month out so we could get excited. Maybe Stitch or Moana could “contact” them and say how they can’t wait to spend time together.