We like Royal Carribean. They have a cruise that is 4 days 3 nights and leaves out of Ft. Lauderdale. It goes to Nassau Bahama's and then to an island owned by the cruise line called Coco Cay. Then return to Ft. Lauderdale. I think there is also a 5 day that includes a stop in Key West.
While in Nassau you may want to visit the Atlantis Hotel. It has a casino and a really nice aquarium to visit, nice beaches. Take a stroll thru the straw market & then visit the shops. There are also tours of the island you can take. Coco Cay has a great beach where you can go on Banana Boat rides, snorkel, parasail and scuba dive. They have a bbq lunch and there are a few shops.
Carnival also has a similar cruise and may be cheaper. Royal Carribean ships are newer and nicer.
As Advisorgirl suggested check the cruise forums for more info. Check online or with a travel agent for deals on cruises.