Your 13 and 16 year olds will love Universal! If they are roller coaster types, take them to Islands of Adventure. If they are more into the movie attractions, take them to Universal Studios. You don't have enough time to do both. Either way, they are the perfect age for this excursion. My boys are 13 and 15, and they loved Universal Studios! The Express Pass is a big help to get past the long lines, but if economy is important, don't buy the Express Pass from the cruise line, buy it at the park for a $25 upgrade from the Standard Pass.
In Nassau, my family loved the Sea Island Adventure (I think it was about $35 per person). They took us on a boat ride to an island where they had activities and games (unlimited rum punch for the adults!). The snorkeling there was amazing, and the beach is something right out of a Bahamas post card. We never snorkeled before, and the boys are still talking about the fish that they saw.
Have a great cruise!