If you're taking the Disney Dream or Disney Fantasy cruise, then you won't be able to miss seeing the AquaDuck, the epic water slide that circles one of the decks of the ship. Riding it during the day, you can see over the ocean for miles, but a hack I discovered is to ride it at night too; the lines are shorter (aka nonexistent), and the ride lights up for a completely different experience than the one in the day, and it's open pretty late.
Tip: Don't put off riding the AquaDuck! The ride closes down if the winds are too high, so take advantage of it while it's running in case you experience inclement weather later and miss your shot. The ride itself is faster and more exciting than I thought it would be; I figured it would be more of a "lazy river" but it definitely has some drops that both kids and adults will appreciate.