My initial impression was that it was pretty touristy right off the port. The shops were cheesy, but it was kind of expected. We booked the catamaran sail & snorkel, which was AWESOME. 1 hour of snorkeling and then you sailed over to a private part of the beach (past the shipwreck) to enjoy the sand and some rum punch. I enjoyed the rum punch a little too much and didn't realize it was a little higher proof than what I'm used to. So uh, lightweights beware! If you are in Grand Turk - scuba or snorkel, definitely, and book ahead of time because they filled up fast.
The best beach of them all. We really loved the desolate feeling of the island, considering there's only a handful of full-time residents here. Rent a clamshell at the beach and enjoy the white sands and lovely water. Snorkel isn't as good here, though. We went on the tropical kayak excursion. It seemed kind of short, but was worthwhile. You are able to kayak in a protected lagoon and see the wildlife - mostly stingrays, jellyfish, a few lemon sharks.
Meh. Wasn't impressed. We didn't book anything here as the cruise excursions were mostly centered around Atlantis (not interested) and the pirate tour was booked up. The city center was cleaning up from festivities the night before from a local holiday and the shops didn't open until late. A bunch of goofball 20-somethings on our ship decided to rent mopeds so the majority of our entertainment in Nassau was watching them stupidly realize they were on the wrong side of the street (British roadways) and getting yelled at by the traffic police.
We booked a tour of South Beach for convenience as we had a late flight and a late incoming flight to Miami (delayed 24 hours) and were not able to enjoy SB/downtown Miami before the cruise. The excursion was convenient and an easy way to walk around SB without worrying about luggage. I've been to SB a handful of times but wanted my fiance to get the experience, as well. Enjoyable. Long, too - time for lunch and a 1.5 hour walk.