Welcome aboard, Colleen! You must be excited to set sail on the newly re-imagined Disney Magic! I was just on the Magic a few weeks ago, for the first time since it's been totally updated and was so impressed with all of the fabulous new spaces! I can't wait for you to experience it yourself!
Disney does offer some great Port Adventures heading to beaches in Nassau, including many that go to Atlantis, but my favorite is the
Blue Lagoon Island Beach Day. I love that gorgeous, remote beach and the great activities they have to offer there. Plus, there are multiple return times which means my family has been able to spend as much or as little time there as we have wanted before heading back to the ship. My daughter's favorite part about Blue Lagoon is seeing all of the dolphins at Dolphin Encounters. What a treat! Plus, Disney takes care of all of the details, which makes us feel secure when we book an adventure through them.
There are a few beaches that you can access by taxi or by walking a bit from the port area on your own. I haven't been to these, as I prefer to stick with Disney's adventures so the ship always knows where we are and we know we won't accidentally miss the boat! But, a few I've heard great things about are Junkanoo Beach, which is the closest to the port area, and Cable Beach which isn't too far from the downtown area and accessible by taxi. You might google a few of the local beaches if you're planning to head out on your own just to see which one most appeals to your family.
I hope those details help as you continue planning your adventure at sea, Colleen! Stop back again if I can help with any other questions! Happy planning!