And also book the princess gathering and frozen meet and greet when your booking window opens. Consider Bippity Boppity Boutique as well and she can look just like a princess.
Sign up for the princess meet & greets as soon as you're able via online check-in.
Beyond that, the must-do's are pretty basic:
Aqua Duck
Midship Detective Agency
Pelican Plunge slides on Castaway Cay (no, you don't need an excursion)
Eat dinner in the main dining restaurants
If you want, you can participate in a Fish Extender exchange, in which you put a holder on your door, and exchange small gifts with other cruisers who do likewise. This is totally optional but she might enjoy it. If interested, you can find more info on the Disney Cruise Meets thread for your cruise, or on the FB link for your cruise.
Also, many Disney cruisers decorate their doors with decorative magnets. Maybe if you can find & order a Disney cruise ship door magnet online (Etsy is one online shop that sells them), she would enjoy that, as well.
Your booking window will open 75 days before your cruise. You can go online and book the Princess meet and greets.
There will also be different character and Princess meet and greets that don't require tickets. They will be listed in the Navigator (newsletter) that you will receive each day or download the app. Take time each day to read the events that may interest you. Here is a snapshot from the Dream listing the characters from one day.
I don't know that there any excursions that are a must. But there are some things you might consider. On Castaway Cay, if your daughter is interested in animals, I would recommend a stingray excursion. You get to feed and pet stingrays and even have a chance to swim in the lagoon with them if you'd like. But, you don't really need to plan anything for Castaway besides just enjoying the beach. Many people in Nassau visit Atlantis, and you can use their beaches and/or plan an excursion that includes their waterpark. If you're looking for something a little more affordable, and not quite so time-consuming, I would recommend going to Ardastra gardens, which is a lovely little zoo, or doing the chocolate tour at Greycliff. Both are lots of fun. You can do them on your own, but if it makes you feel more comfortable you can also book both on a tour through Disney.
The biggest "must do" is - relax and realize that you're not going to be able to do everything. You'll have a much nicer experience by not trying to do it all and not being able to.
I agree, if Princess meets are important, be sure to book tickets for the Princess Gathering and the Frozen Meet & Greet when you activity booking window opens.
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