I spent several days in and around Zadar a couple of years ago, and loved it (and Croatia in general). I’ll think about the one-day limit and come back to the thread. For now, I wrote some general thoughts on the country, and some specific thoughts about Croatian food, in this thread.
I sailed past Santorini on my way to a smaller, less tourist-crazy Greek island. My only real recommendation there is that the port areas of these islands can get really hectic, so do not try cutting it close when it’s time to re-embark. Give yourself plenty of leeway.