The beauty of a first cruise is not knowing the things you are missing! I knew nothing about cruising when I booked my first sailing on the Wonder 15 years ago - at 3 weeks before sailing.
It also helps that you are just going to the Bahamas. Not a lot of things to book, anyway.
You are going to get the essentials of a
Disney cruise. You can try all the standard meals in each of the three Main Dining Rooms (which one on which night is assigned, as is time. You will probably be second/late seating). If you do a 4-night cruise, you will also have a Pirate Night menu. (You get the fireworks on the 3-night cruise, too, just not the menu.
Unlike a lot of cruise lines, you do not have to rush onboard to make entertainment reservations. (unless drinking in the hyped Hyperspace Lounge is important for you.)
On
Castaway Cay, loungers are plentiful and they are complimentary (including the umbrellas). If you want to ride a bike, they are typically available to rent once on the island. Parasailing is probably sold-out. Jet skis have not been available to rent for several years. Just treat it as the private beach day that it is. Have some included lunch and enjoy the beautiful Bahamian water.
Have a great cruise!