Great ideas here! I am getting married in WDW in April 2015, and we are looking at the Grand Floridian to do a pre-wedding Mom and Daughter mani-pedi before Afternoon Tea. It's really expensive, but it's at the top of my priority list, and something I've dreamed about doing for a long time. I think a lot of the ideas and comments here are great, especially the one about how to sneakily mention you've been looking at stuff online and think it would be cute for her. My fiance does this all the time, so if he were to try and fool me with something like this, I probably wouldn't catch on, especially when it comes to getting excited for a Disney trip by sharing Disney things you find online with each other. The Grand Floridian Senses Spa, or Mandarin Spa at Swan and Dolphin, or whatever the one is at Saratoga Springs will definitely give you that "Disney experience," but with a price. I also don't know much about the services they include (but my fiance is calling sometime this week to find out!) Also, there's always that worry that she might sporadically decide to have her nails done this one time before the trip (maybe expecting a proposal is coming soon and anticipating it by making her nails prettier than usual). I think whatever you decide to do will be perfect, whether you get her a gift card before hand for a nail salon close to home which is nice because not only will it be more affordable, but it also wont take away any precious Disney Vacation time if you are the type to try and cram in as many activities as you can in the trip, and it is a great idea to use that "I saw this online" idea. Maybe slowly plant the idea by sending her cute pictures of nails you see via text message once every couple days, etc. The Spas at Disney are also amazing for the sheer purpose that it's sort of a once-in-a-lifetime splurg, and an engagement is a perfect excuse to spend that extra, right?
Also, my biggest tip is get her to do the shellac on her finger nails (I agree with the above comment about not doing it on toe nails). It will last for weeks, the only downside is it is usually more expensive than regular polish, and she will have to go back to a nail salon to have them remove it because normal nail polish remover wont take it off (they usually charge $5-$10 just to remove if she isn't getting more done).