Cruises are a great way to go on vacay and actually know what the cost will be beforehand if you follow a few basic rules ...
.... keep "excursions" to a minimum. the absolute best time I ever had was when I filled my backpack with stolen breakfast goodies from the ship and hit the town with a native guy in Aruba that I met on the beach. I had the food and he knew the town. We had a great time just hanging out together.
.... kareoke is fun on a cruise. Get up and do it!
.... sometimes it is best to just stay on board, especially if you just want to chill by the pool and read ... etc.
.... enjoy the meals.
.... walk the deck for a good 20minutes after every meal listening to the ipod. You will actually come home with weight lost!