I don't think we've ever been on a cruise shorter than 7 days except one 4 day one many ears ago. when i was working, it took me 4 days just to get unwound enough to start enjoying the cruise, and now that I'm retired since airfare is such a large part of it 10 to 14 dayers are the norm.
You can pack lighter because you can do laundry one day to get the underwear etc.
Take more film/memory sticks.
Don't know whether you will have one formal night or two, but for men if two, just take two ties.
Relax right away so you can enjoy each and every day