You can walk around the the Atlantis resort and casino for free but if you want to see the aquarium there is a charge. The aquarium is nice but not worth the money if you ask me.
Cabbage Beach is a short walk down from Atlantis or you can just take a taxi there from the cruise port. You can rent lounge chairs $10 for the day, we got 2 for $15. There was also a guy selling rum punch served in a real coconut. It was $10 for all you could drink. What a deal.
We love Cabbage Beach, very relaxing and a lot of fun. Hope this helps.