Group students into pairs, and have them place their desks together in a way that forms a large circle in the classroom (or clearly-defined rows.) Pass out the assignment you want students to complete and have each person in the pair choose to be Partner 1 or Partner 2. Set a timer for an appropriate amount of time for each pair to complete the first question or problem in the assignment, then sound a signal to let all the Partner 2's know to move one seat down the row or clockwise while the Partner 1s stay in place. The new partnerships then work together to solve the next problem. Halfway through the assignment, announce that students should switch roles, and now the Partner 1s will rotate while the Partner 2s stay in place.
Like other experienced teachers, I've taught the same concepts and skills to students so many times that I've developed a huge repertoire of activities that I can build into my instructional time. I have activities that provide more scaffolding if the kids didn't really seem to understand the lesson, and activities that will extend student learning if what I taught was too easy. I have activities that will take a long time if my lesson ends early, and activities that can be done quickly if my lesson takes too long.