The best excursion is the cruise itself, don't worry if the way in which you are budgeting your trip is amking you get concerned about the costs listed for the formal excursions booked through
DCL. You can have a great cruise w/o any of them, as we have on 4 DCL cruises (myself, DW, both now in our *ahem!* early 40's, and DS, now 9). That said--you can find many, many DISers who have had a great time on many of the excursions, and many threads exist about those excursions.
In Nassau, for a beach day at a modest cost, you can take a cab, or even a bus, to the Wyndham and use their beach and pool, and lounge chairs, but do bring your own towels. You can find threads about this, including how to find the bus. On our last cruise it turned out to be not quite warm enough, and way too windy, to want to go in the water, we'd already wandered around the downtown area (and not been impressed) on a previous cruise, so we never got off the Wonder. And enjoyed a ship w/o crowds!
On CC getting a float for the few dollars rental fee is worthwhile, but in case you want to wait n see how the weather is that day for how much water time you will choose, you can make that arrangement right there at the beach and need not book in advance.