I would definitely check out
disneycruiselineblog for navigators of previous cruises as this will give you some ideas about what is going on onboard. There is so much to do and you don't want to miss out on something because you don't spot it.
Also I would try and avoid booking too many things. Nassau is OK but as its your first cruise you may want to not bother with a tour and just stay onboard or do shop port shopping. You can look at the excursions DCL offers online and see if there is anything that grabs your attention.
In Castaway too I would say there is enough that you don't need to do an excursion although if you are active then you might want to do the snorkle rental or the bikes (we did the getaway package on our first visit which included both and was good value).
Most food and soft drinks are included but remember if you order a soft drink at a bar they will charge you. On the pool deck there are drinks stations where you can get soft drinks and take them anywhere on the ship so worth grabbing one before a show to save a few bucks. Food wise, any snacks by the theatres are not free and also some snacks in the bars. There are however plenty of free food options including free room service (you just tip for the delivery a few bucks).
You can take 2 bottles of wine onboard to drink in your room if you want to (or 6 beers) otherwise alcohol is chargeable but not as expensive as in Disney World. They also do drink tastings which you can book and these tend to be great value for what you get if you like this type of thing.
Meal wise, you get to rotate around the restaurants each night so you get to try all the menus. There are however 2 restaurants that you pay extra for and need to book if you want to dine at them. Remy and Palo are both adult only and do brunch and dinner. I always think brunch is nicer as then you don't miss one of the main restaurants for dinner.
Your dining time is either early (5:30ish) or late (8:30ish) depending on what you book. If you want to change it its best to try and do as early as possible as sometimes there is a wait list. With either you still get to see the evening show as they run this twice at the opposite times. This means if you are on the 5:30 dinner your show time is 8:30 and if you are on the 8:30 then its 5:30.
Im sure I will think of other things once I post this but that's the most important things I think
