Atlantis is the best water park ever! There are so many different kinds of water rides, slides and play areas for all ages. My family of five (kids ages 8, 7 and 5) vacationed at Atlantis for one week and I felt that we needed more time to fully enjoy the resort-it is massive.
That being said, since Atlantis will be an excursion with a limited time on shore, I would check out any of the following: the amazing huge aquarium in the Royal Towers, ride the Rapid Rivers (lazy river with strong currents), drop down the Leap of Faith slide or just visit many of the different pool areas in the resort. Have a wonderful time!