I'm not sure if this qualifies since we will most likely do it again, but the one thing my wife and I wish we had done differently on all our cruises is stay up later.
We tend to keep our normal schedule of up by 3 or 4 am....which means we follow our normal schedule of going to bed early.
It seems like every day we see things in the cruise planner that we want to go to that night, but we fall asleep.
On the flip side, the ships are pretty much deserted that early in the morning.