In order to make the most of transitions, we sing, count, recite poems, etc. I purposefully give them something to do, but in order to use the time effectively, I have to be prepared and know what I'm going to expect of them. Here's how I make that happen. I have a section in my weekly lesson plans for what I want to do during those times. (Does everyone already do this? If not, I'd highly recommend it.) I do this on a week-by-week basis, because the things I want to do differ depending on the time of the year. This is a picture of the transitions I was doing during the second week of school.
Whether you understand what all those transitions are or not isn't the point of course. Staying organized and teaching with a sense of urgency is. It's sure helps me!