we flew into Miami the day before our cruise and stayed at a hotel near the airport. They picked us up and took us to the hotel, then they bought us back to the airport the next morning. Our hotel had an evening "reception" (Monday - Thursday) which was sufficient for our dinner and a very nice breakfast buffet in the morning. (Hyatt House, Miami Airport)
From there, we took the
DCL bus to the port. It was very easy. There might be less expensive ways of doing this, but it gets us in our
Disney Cruise Frame of Mind without the hassle of getting ourselves to the port. The people waiting for us at the airport were great, the bus trip was enjoyable - it was our first time in Miami and we kept watching the video about our upcoming cruise (our 10th dcl cruise) instead of looking at the city. The bus people took care of our luggage at the port so no hassle there (just had to get it to the bus at the airport). We will be flying into Fort Lauderdale and will drive ourselves to the Miami airport and take the bus again - and we are just two adults
The bus ride was between 20 and 30 minutes.
I assume that you have already completed your Online Check In for the cruise. If not do it As Soon As You Are Permitted. Fully completing this task will give you your arrival time at the port (we like to board as soon as possible!!). I believe that we had an 11:00am arrival time. DCL said that we should get to the airport between 9am and 11am (since they were going to be really busy later with people flying in the same day (don't do that if at all possible - to many things can go wrong!)
The cruise terminal in Miami is a little small - So I believe that they are going to be a little strict on arrival time if you are getting yourself to the port (driving, hotel shuttle, etc). I think they are a little less strict if you are arriving on their buses from the airport. There were photo ops with the characters while waiting to board. Lots of places to sit, but it can get crowded. Lots of people using their electonic devices while waiting (iPhone/Pad, Nook, Kindle, handheld games). Boarding usually starts around noon. Oh yea, my family tries to wear "complementary" clothes for the boarding picture that they will take. Photos on board are expensive, but they usually do a good job so you will
want/need to buy some.
Upon boarding you can have lunch at Beach Blanket (pool deck, some inside and outside seating) or Parrot Cay (deck 3 aft), inside seating. Rooms will be ready to occupy sometime around 1:30. Pools are usually available to use upon boarding, so bring your swimsuits in your carry on (or the kids can wear theirs under their clothes until it's time to swim.) This will be the least busy time at the pool for your entire cruise.
Key West: small area where "everything" is. Easy to walk. We did NOT do a DCL excursion. We pre planned (but did not pre buy) to do the "Hop On Hop Off"
TROLLEY not the train. (if you get this through the ship you do not get hop off/on privileges). The entire circuit was 60-90 minutes. Very enjoyable and informative. There's an aquarium and a shipwreck museum on the island that might keep a 5yo occupied. The Hemmingway House will not be interesting to the kiddo's BUT the 6 toed cats that live there might be. The Truman White House was very interesting (for adults not so much the little ones). We had a great guide.
Nassau: This was our second trip to Nassau: didn't get off of the ship. I don't know what the "packages" are for Atlantis at Paradise Island. On our first trip there we visited "the Dig" a Lost Continent of Atlantis themed aquarium. I remember it was awesome. I don't believe that there is a ship excursion that includes "just that" anymore. The water park is expensive (170 per person - and I though Disney parks were expensive) and it might include the Dig. Consider treating this as a sea day and enjoy the sun and the pool (again will be less crowded).
Castaway Cay: Great Beach Day. I would rent a mat or float once you arrived on the island - don't pre book it. Remember, you are not "stuck" with your choice of float or mat for the day - you can drag it back to the rental place and trade if you want too. I don't have any experience with the kids clubs on the island - my kids never participated while on castaway cay, they loved the clubs on the ship - and I think they are different now than 10 years ago.
Last day of the cruse sucks, NOT Castaway Cay - the packing up your room afterwords. There is still lots to do but you have to get your luggage packed, tagged (housekeeping provides these for you) and out into the hall by 10pm. Next morning after breakfast you "wait" until your character is called and then you can disembark the ship and find your luggage, get through customs (don't take any food off of the ship with you) and you will be on your way.