No matter what cruise line you choose, no matter what itinerary you choose, there are, from my perspective, two MUST DOs.
First, be on deck, or outside on your balcony as the ship begins to pull away from the dock on the first day. For me, it's almost nirvana.
Second, whether you are a late night party animal or early to bed geezer, be sure to get up just before sunrise the first or second morning, and watch the sunrise from the deck or your balcony, with a nice cup of coffee (or tea, or Ramos Fizz), and your honey. The moment is forever etched in your mind as a highlight; if it isn't a special moment, then you don't need to get up early again...but do it once, early in the cruise, so you can keep doing it if you like it.