We recently did a cruise on the Disney Fantasy out of Port Canaveral. I got SO MUCH info from www.disboards.com in the cruise line forum. You have got to check it out. Seriously, you'll get all the info you need.
When are you going?
Dinner on a Disney cruise is generally more casual than on many other cruise lines, so you don't need to bring a ton of fancy clothes for dinner. One dress for formal night and you can wear jeans with a nice top the others. Most ships have one pirate night, where you can dress like a pirate if you want to. It's a lot of fun.
You can bring an item for the characters to sign. Many people do pillowcases, t-shirts, or photo mats. I brought aprons for my kids. You bring it to guest services, along with sharpies (get fabric sharpies if you do something made of fabric) and you'll get it back on the last night of the cruise with tons of characters signatures. If you include some candy in the bag with your item and pens, you'll get more signatures than if you don't. :) Maximum of two items per stateroom.
Castaway Cay is awesome! If the snorkeling is reopened (it closed following the hurricane), there is a sunken ship you can explore. I didn't know about it and missed out on the experience, but I hear it's a lot of fun. The rest of the island is really fun, with waterslides, bike rentals, snorkeling, and more.
Make sure you pack a carry on bag with the stuff you're going to want your first afternoon on board (maybe even bathing suits). You often don't get your luggage until early evening.
Visit the kids club during the open house times so you can check it out. Sign your kids up as early as possible so you can send them whenever you want. If you have a child under three, book your nursery hours in advance because they do fill up.
Have fun!! Wish I was going with you.