It's actually quite easy to take just a carry on and one personal bag for a one-week cruise. Most people take so much unnecessary junk, and it's expensive!
This is plenty for a warm-weather cruise:
5 shorts outfits (ideally, bring light cotton tops that you can wash out in the sink and hang to dry . . . and if the shorts are too dirty to wear twice, send them to the laundry -- the cost will be less than the checked bag)
2 dinner outfits & accessories (make sure both match the same pair of shoes -- this is enough for us because we typically only eat in the MDR 3-4 times for dinner, and we don't do traditional dining . . . so we don't sit with the same people)
2-3 swimsuits and coverups
Pajamas, under garments
One pair of comfortable sandles in brown or white
One pair of flip-flops or pool shoes
Toiletries, including plenty of sunscreen
Medicines you might need
Books, magazines, reading material