In my experiences, there is always a calypso band playing island music and the woman loooove that. Stay close by, and you will be rewarded. At night you can go to a Broadway style show, or hang out in several different cocktail lounges. Also, there are usually one or two dance clubs on board the ship that are open till the wee hours of the morning. Most likely on a four day cruise you will spend some time docked in a place such as Nassau in the Bahamas or Cozumel in Mexico depending on which cruise you take.
Most standard cruises are inexpensive.
That includes all of your food, and entertainment while on the ship. Drinks are not included, but hey, life's not always fair. Unfortunately, if you don't live in the Miami-Ft. Lauderdale area you are most likely going to have to fly. When you go to your travel agent, they can set you up with a flight and shuttle bus that will take you directly to the ship.