The day at Atlantis was priced reasonably, though Atlantis is still expensive. You can go to Atlantis and get a day pass from their ticket booth outside for $135. The Carnival excursion was $180 but included transportation and lunch. It was a reasonable premium, but next time I would take the cab to Atlantis. I can't believe they would ever sell out.
The beach is very pretty here but we are more the snorkeling/diving types. In Half Moon Cay we got out and snorkeled from the beach. There is a baby reef near the private cabana with the slide near the pier. There is supposed to be a better snorkeling spot at the north end of the beach but that is a really long walk, so we didn't make it out there. I would recommend taking their boat snorkeling excursion as there was very little to see snorkeling from the beach.
In Grand Turk, all the snorkeling excursions were sold out (Xmas cruise) so we walked up to the north end of the beach near the next pier north of the cruise dock. Snorkeling wasn't bad their practically from the beach. Still I would take a cheap boat snorkeling excursion next time.