I am not sure about Belize but I just did the one in Honduras. THAT WAS A BLAST. A couple of things though. First, if it was rainy, make sure you don't wear a nice shirt. My shirt got all kinds of dirt/oil on but but it was SO worth it. Second, after the first couple of runs, try to look around! Me and everyone else was talking about concentrating on the line and not the view. Trust me the view is nice!
Jan '08 - Carnival Legend
Aug '04 - RCCL Rhapsody of the Seas
Apr '02 - RCCL Adventure of the Seas
Dec '00 - RCCL Grandeur of the Seas
Mar '96 - Carnival Imagination
Mar '94 - NCL S/S Norway
Didn't book through the ship, but through a private tour company there. (searched the web; can't remember the name right now). We did the zip-line canopy tour and cave tubing in Belize and it was the most awe-inspiring excursion I've ever done. (I'm 49 and it was my 10th cruise; on Carnival Valor). We were with a group of about 10 (four in our group) and we were taken by van to the mountains, and we got to the zip line area LONG before the cruise tours arrived en masse. Then we went to the cave tubing where we got our tubes and took off on the most peaceful, serene and beautiful trip down a river (long walk to where we started; carrying our tubes with us), but when we ended our (longer than the cruise tours) river ride, we got out and saw HUNDREDS of Valor cruisers just getting into the water.
I highly recommend researching this group, as it was the best of my trip.
I've been to Honduras on the Valor 2x, and the first time we spent the entire day at the Sante Day Spa (find them on the net). They are an American couple who built a house there and offer full spa services for ridiculously low prices. All day, lunch and transportation to/from the ship is arranged. They only take 8 people per day when cruises are in port, and they offer snorkeling and kayaking with their equipment from their private beach.
The second time we were in Roatan, we hired a cab driver (in advance, again search the internet and web forums for receommations) and he took us all over the island, including to his cousin's house, which is adjacent to Anthony's where they have dolphins etc. It was fun to watch all the hoards of people off the ship, and we were just relaxing with our new native friends. Don't miss the iguana farm and the cameo factory!