Sorry, can't help you with the Dream, but as for Orlando to the pier, we usually prefer to rent a car one-way. I'll drop Nancy off at the pier with the luggage and then drop the car at the rental company where they provide free shuttle to the pier. Then, when we come back from the cruise, we do the same thing in reverse. This gives us the option to do what we want when we want and it's usually cheaper than the cruise line transfers. This is especially true if there is more than two people. If you have the time, I suggest going to Kennedy Space Center, which is amazing and not too far out of the way.
As for Roatan, this is one of my favorite places for snorkeling. West End Beach is amazing! The first time there, we did the zip line beach stop through Shore Trips. Found out the zip line they use is much longer, better, and safer than the one Carnival uses. Plus, we got to spend more time at the beach than the excursion offered by Carnival.
As for Belize, unfortunately this is one port we've not yet been to, but have had alot of clients and family go there. The one biggest excursion everyone wants to do is the cave tubing adventure. Again, this is best done through Shore Trips instead of Carnival. Our daughter and her husband just did this and they said there is a big difference in the two and their experience with Shore Trips was much better. But Belize is like any other place, including Washington, DC. It's a safe place if you remain in the tourist areas or do planned excursions. But if you go off the beaten path where there's no tourists, then it can get a little tricky. Don't listen to what people say about it being unsafe, as that's too general. Belize is known for its outstanding scuba diving. I had a friend who owned a scuba diving company down there for many many years and was always trying to get me to come there to teach. He owned a house and worked there for a long time and never had any problems. He was always sending me pictures to entice me and it's absolutely beautiful. He finally decided to sell the company and retire there.
As for Costa Maya, I'll be honest and say it's not one of my favorite places. Not alot to do. Beaches are nice and snorkeling is good, if you find the right place. We prefer to go to an all-inclusive resort for the day and get away from the crowds.
As for excursions, we really prefer to do independent ones as they may be the same price, or even less, but they're always alot less crowded and usually a whole lot better. Don't mean to sound like I'm pushing Shore Trips, but we've used them for years and have always had a wonderful experience with no problems whatsoever. There are several other really good excursion companies out there - I'm just not a fan of excursions offered by Carnival for many reasons.