Keep receipts for anything you buy outside of the United States. Don't bring Cuban cigars home unless you want to run afoul of the law. Enjoy them on the ship. Take as few clothes as possible and make sure most of them can be washed by hand and dried quickly. The Century had no self-service laundry, although some newer ships do.
Most ships offer dry-cleaning service.
If you're prone to motion sickness, be prepared with Dramamine or ginger tablets.