When I'm home I'm it seams i never sit down with a book to read. On cruises with sea days I take my tablet and sit down in the serenity deck and read a good book. I sleep in, have a nice slow breakfast (may splurge and have fruit juice and coffee, may even have a danish) I walk laps of the ship to get a few miles in and hang out with friends or family that I may be traveling with. We will also do some of the ship activities like Trivia, watch the glass blowing show, attend some of the lectures (Celebrity had a great lecture series on the rise and fall of German U boats during WWII), wine tasting seminars and enjoy some of the live music.
Sea days are some of my favorite times on a ship to just relax. On my transatlantic cruise from Rome to Florida I did get up at 5 am and watched as we cruised through the Straights of Gibraltar.
I pick cruise lines differently for short cruises and for long ones. I like Carnival for a 4 night party cruise but for a 2 week or longer I love Celebrity for the lectures, wine tastings and more relaxed high end experience.