Cruise casual gets more casual as the cruise goes on, especially if its a longer cruise. On our 11 day Med cruise there were a lot of jeans and t-shirts towards the end but at some tables people were still quite dressy. Packing for 11 days (plus more in Barcelona) is hard so I brought 2 dressy black pants that I just alternated with different dressy/ casual tops. I brought a nice dress for formal night and wore the same one to Palo(happened to be semi formal that night anyway. I had some comfortable capris that were fine for dinner too, with a nice top and also wore them for excursions (knees and shoulders need to be covered going into any of the churches).
One evening we were just back from our Disney excursion in time for the 6pm dining and we just went straight in as we were(t-shirts and capris), too exhausted to change, and it was fine.
Really what ever you're comfortable in, is what you should wear, while at the same time respecting the no swimsuits/tank tops rule. Remember too that you need to carry those heavy cases at some point too!