Hi Elizabeth,
How wonderful to be planning a first surprise visit to Walt Disney World! Be sure to stop at Guest Relations at any of the theme parks to pick up a "first visit" button for your daughter to wear. Also, if she is unsure about the characters, I would not push the issue of making her meet them up close. Since she likes the princesses - focus on them!
Many of the opportunities to meet princesses are in Magic Kingdom, so plan a day or two there. Some unique princess meetings at Magic Kingdom include meeting Ariel in her Grotto, and meeting Merida. When we met Merida, my daughter was able to shoot an arrow and do some coloring before her turn to meet Merida. Also, be sure to go to Enchanted Tales with Belle, where Belle retells the story of Beauty and the Beast with help from the audience. There are also lots of opportunities to interact with princesses in Epcot. You can meet Mulan in China, Belle and Princess Aurora in France, and Elsa and Anna from Frozen in Norway. Please know that character appearances are subject to change. Be sure to check the Times Guide at each park for specific times.
Another way to ensure that she will have great interaction with the princesses is to book a character meal. The most popular of these for princesses is Cinderella's Royal Table, located in Cinderella Castle in the Magic Kingdom, or at Akershus Royal Banquet Hall in Epcot. Be sure to book these 180 days in advance if you can as they fill up quickly! A final idea for a really special treat: consider
My Disney Girl's Perfectly Princess Tea Party. At this fancy occasion, little princesses have tea and enjoy a special visit from a Disney Princess.
Elizabeth, I am limited in the amount of space that I have to answer. Please do not hesitate to ask if you have any further questions!