No fee to get in the Casino, and you can self tour most of the grounds without any cost. The beaches require admission. Atlantis is used to people walking through, checking out the premises. Be sure to swing by the restaurant, they have a whole wall set up for viewing the aquarium while you eat. The reflecting pools outside make great places for pictures with family, and friends.
If you are young, you can walk from the port to Atlantis in ten minutes. If not, there is a water taxi right from the dock, or you can grab a taxi for a few dollars ( I'm thinking 6) per person. Don't quote me on that one. If you get a chance when you get back, hit the straw market, just a couple of blocks from the dock, and try Senor Frogs for a cold drink before getting back onboard.