Pack like you would for any resort. Cruise vacations are casual by day, whether you're on the ship or ashore. In the evening, ships vary as to dress. As on shore, attire is dictated by occasion.
Cruise ships are like floating resorts with all the same entertainment options.
You can relax by yourself and lie back in a lounge chair, breathe in the sea air, soak up the sun, read good books, or watch the ever-changing view. Or, you can join in exercise classes, dance classes, sports contests, and other organized deck activities. Perhaps you can practice your tennis stroke or golf swing, or shoot some baskets. You can go for a swim, stretch out in the sauna, or work out in the gym. You can see a feature movie, attend a lecture by renowned experts, or play backgammon or bridge.