It was mentioned up thread, but I also wish I'd brought an extra pair of yoga pants. I found myself wearing them at least twice a day. Once when I got up before everyone else and wanted to go walking and get coffee and usually again mid afternoon between daytime activities and dinner. It was only a 4 day cruise, but wearing them twice a day meant I really needed a second pair.
Random things I threw in and will no longer travel without: small garbage bags for laundry, extra ziplock bags for purchases or wet swimsuits, scissors, and blue tape to make sure toiletry bottles stay closed. I actually now just have a bag with all that stuff in it so I can toss it in my suitcase as I'm packing.