Welcome aboard Akshay! The Disney Dream is such a marvelous cruise ship with so much to offer! You will surely have a magical time with your wife and little one!
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Discover Atlantis Port Adventure is a great one, as it allows you special access to the Atlantis resort, which is a place not to be missed in Nassau! You said you've chosen the 11:15 adventure, so once you spend a few hours there, you will be able to decide when you're ready to head back to the port. Shuttles run every half hour from 1:00 - 5:00, so simply head back to your pick-up point when you're ready.
It's fun to explore the straw market and the forts on your own. Fort Fincastle is a great spot with an amazing view, and I believe it's only a dollar to go in. It's a short hike up from the straw market or an even shorter cab ride away.
Before you leave the ship that morning, be sure to note the all aboard time, so that you ensure you leave yourself more than enough time after your tour to explore Nassau on your own before heading back to the ship. Sometimes lines getting back on the ship just before the all aboard time can be long, so keep that in mind.
Have fun planning your magical cruise, Akshay!