Welcome back, Theresa! There are so many fabulous options available for Port Adventures in Nassau.
The good news is that if you choose one of Disney's Atlantis Port Adventures, there are multiple return times, and guests are able to choose when they want to return to port. The return transportation from the Atlantis Resort runs every half hour beginning at 1:00 p.m. and ending with the last transport back to the port area at 5:00 p.m. Once you've enjoyed your time at Atlantis, you're welcome to head back to the port area and spend the rest of your day shopping and exploring the downtown area of Nassau.
There are plenty of shops to visit just outside the port and the Straw Market too, that while a bit hectic, is something to be seen for sure. Just be sure you have cash on hand for bargaining and you know that some vendors in the Straw Market can be a bit pushy. If you know that going in, it's a fun experience and you can score some decent deals by bargaining. My kids got a bit nervous at first with the vendors calling out to them there, but now they expect it and know that they can just stick with us and we can still have fun browsing. Besides the Straw Market, you'll find plenty of other stores to enjoy in the area, all very close to the ship.
If you're hoping to visit Atlantis, spend some time shopping downtown, and enjoy a beach, you will probably want to choose the
Atlantis Beach Day which gives you access to the beach at the Atlantis resort as well as the "ruins" of the archeological dig and the aquarium. Some of the other Atlantis Adventures do not include access to the beach there, such as Discover Atlantis. So, be sure to choose the right adventure if you want to have some time on the sand too!
Have fun planning, Theresa!