Snorkling, Fun Holiday for an afternoon (great beach and pool - they can pop from 1 to the other), Blue Hole (either one), Abeukota (?). It's a long way to go from Negril, but Glistening Waters (in the evening) is awesome; there's also the Cool Runnings Water Park. Your 20 year old may want to try scuba diving - maybe even the 10 yr old - Dream Team Divers (their shop is at Sunset on the Cliffs, and they are very professional and would not let the kids just go! They will get instructions and be supervised, totally!).

Again, a long way from Negril back to Mobay, but Mobay Undersea Tours.