Hi Lacey,
Thanks so much for asking your awesome question here on the Moms Panel. Surprising kids with a trip to Walt Disney World is one of the most magical things we can ever do for them. I still surprise my kids with vacations to our home away from home, and it's always just as exciting as the first time!
For your sweet little princess, I suggest putting together a little gift package to open as soon as you depart the plane. Since she loves Disney Princesses, a Cinderella or
Ariel Ear Hat and an autograph book ordered from The Disney Store would be perfect! Place them in a nice gift bag, and put at the top of your or your sister's carry-on. This will make it easier for TSA to look in the bag (and not ruin the surprise) and for you to grab the gift as soon as you land.
Have a wonderful time planning your surprise vacation, Lacey! If you have any other questions, feel free to ask us anytime. We're always here to help.
Have a fantastical Disney day!