My husband and I sailed NCL in April 2002, same stops...
Roatan...we did the boat's shore excursion to Tabayana Beach, beautiful beach, a little crowded though, mostly NCL cruisers. I think when we went on our cruise, Roatan was a new thing. I'm not sure if things have changed in Roatan, but I would recommend the boat's shore excursions, especially with young children. The area the boat docks is a concrete pier connected to a dirt road. Who knows, by now, it may be highly tourized, which would be a shame!
Belize...we did some shark/ray alley snorkling tour. The snokling was okay, however, it is nowhere near land. The beach we went to afterward was abosutely beautiful and very quiet, the only people on it were the fifteen or so of us that did the tour plus a few of the resort guests.
I hope you enjoy your cruise on NCL....we had a lot of fun!