Congratulations on your first cruise. The Fantasy is a favorite ship of mine and is one of Carnivals top rated in service. For a first timer I'd recommend the excursion to the Atlantis or the Blue Lagoon island. Either one is a pretty good time. Nassau is o.k. but not anything great. I believe the market is back to limited operations after the fire. Others may be able to give more detail.
One of my favorite places on the Fantasy is out the rear door of the Windows on the Sea Grill where there is a small covered patio where you can have drinks and watch the wake of the ship. It's quiet and peaceful there.