Hi Jennifer!
My daughter is 5 years old, too! We went for our first BIG all-family vacation in September and she had the time of her life! I think the Boutique and breakfast with the princesses will be a highlight for sure!
One little thing that really made my daughter's vacation special was that before we talked to the Princesses (we did the breakfast at Akershus), we thought of a question or something else to tell each of them. So, she told Belle that she loved to read just like her. She asked Ariel where Flounder was. When we saw Mulan in Epcot she asked where the Lucky Cricket was. And every time we initiated a conversation, we were rewarded with a fun (and usually long) conversation in return! My daughter was in heaven when Mulan showed her the hidden lucky cricket in her dress.
We didn't do autographs (maybe my bag was too full to fit in a book and pen, too?) but I saw several people with the Little Golden Books for each Princess (or other character) and had those autographed. For my book-loving daughter? That would be awesome!
My daughter loved wearing her Mickey ears hat, her pin trading necklace (we usually just buy one pin per trip!) and bringing all her Disney stuffed animals to the hotel. You could do a Disney-themed vacation countdown before you leave. Or have small special Disney toys (from the Dollar Store or Target) as gifts each day (saves on souvenirs!). There is something about all that Disney-themed stuff that just seems cooler when you're at Disney World!
Have so much fun with your daughter!
Let me know if you have any other specific questions!