Calabash Eco Adventures is a great outfit for tours on Grand Bahama Island. They offer a number of guided sightseeing and driving options to go along with some of their more active trips. Drop them a line at http://www.calabashecoadventures.com and see if they might be able to customize something for your parents.

I've been to Freeport and can definitely say you should get away from the port area, which is very ugly and industrial. There's a six-hour Jeep tour (about $120/person) that comes highly recommended. It takes you around Freeport and includes a lunch stop at the pretty Garden of the Groves nature reserve and another hour-long stop at a quiet beach. You can also visit the Garden of the Groves on its own if your parents don't fancy hopping in and out of a Jeep all day. They recommend planning about three hours to visit the preserve; entry is $15/person, plus they should factor in the cost of transportation to and from the gardens. Check on the cruise ship about pre-booking tours for both the above activities. Sorry I can't help on the Nassau front!