My wife and I and my parents went on a 7 day cruise on the Carnival Glory out of Miami to Cozumel, Belize City, Isla Routan, and Grand Cayman.
The main piece of advice that I will share that I learned from this trip was to not get a room that is in the lower aft (back) part of the ship. This is where you feel the most boat movement and also this is the noisiest place on the ship from the engine and the propellers.
The boarding process was smooth. My advice to any new cruiser is to just follow the instructions the Carnival folks give you and be patient. Many people seemed to try to skip some steps in the process and ended up getting frustrated while they waited in line when it seemed we all should be relaxing and waiting for our turn.
1st day - Sea day
This day was full of fun events all around the ship and plenty of time to sit on the upper decks and enjoy the sun. I liked the day at sea because it gave me a chance to just explore the ship and see different activities and shows that were going on in my own leisure.
2nd day - Cozumel
My wife and I walked off the ship and got a taxi to Paradise Beach. This was not a Carnival excursion and we only paid $23 after spending 4 hours on the beach and having some lunch. Google paradise beach Cozumel and see if it is something you like, and I highly recommend it.
After the beach we took a taxi to downtown and just walked through some of the shops and my wife had fun negotiating deals with the locals.
In general, Cozumel is a place where you don't have to make a Carnival excursion to have a good time.
3rd day - Belize City
I will start off by saying I do not know why Carnival stops at Belize City. My wife and I again didn't make a Carnival excursion plan and we tried to get off the boat and find something on our own. There is nothing in Belize and if there is one place you really need to do an excursion, it is Belize. There are some shore shops that sell diamonds and jewelry, but you are really limited to an area of a few blocks.
4th day - Isla Routan
My favorite place. My wife and I book a Carnival excursion to Maya Key private beach island and it was awesome. Honduras looks like Jurassic Park, but only with beaches all around it.
Even if you did not plan an excursion, there are some really nice shops\restaurants and a beach you can walk to.
I mention this to say that this is usually the worst night on the cruise in terms of sailing conditions. At this time you are leaving the cost line and cruising in the open sea so the waves are little bigger.
5th Day - Grand Cayman
This place is gorgeous. My wife and I got off the boat and asked some taxi drivers what were some nice beaches. They took us to a beach club on 7 mile beach (its one long beach, with several beach clubs on it) and it was $30 and well worth it. the water and sand was like a post card.
Grand Cayman also had probably the best amount restaurants right and the port and there was some good shopping as well.
6th day- day at sea
This was the sad day of knowing you are going home, but unlike a regular trip where the day is just travel, you still get to have fun. This day has more fun things planned than you can fit in. just figure out 4-5 shows and events you want to go to and you will forget that you are traveling back home.
7th day - port
Getting off the ship was very very painless.
We did self-departure so that meant we carried our own bags and were the first people off the ship. We did this because we had a close call for a plane to catch. If you don't need to do this you can set you bag out the night before and you get off Sunday am at a leisurely pace.