Nassau is not one of my favorite ports so I am not as objective as some may be, but I think the Acquarium might be one thing to consider. Other than that we have found going to one of the major hotels and casinos for a few hours can be fun. To be very honest, after about 4 or 5 times there we get off the ship, do a very little shopping and return to the ship to enjoy the pool when many others are in port. It is a very relaxing way to spend a few hours.
The excursions for your sailing (I think I remember it's next April???) won't be up on the NCL site until about 2 months before your cruise.
Patience is a virtue (Just keep repeating that to yourself, over and over, as I am . )
We cruise in December and I'm still checking every day to see if they put the excursions up early .
"A truly happy person is one who can enjoy the scenery on a detour."