In my classroom, we happen to transition from lunch right back to the classroom. Before moving onto grabbing our math workbooks, we "Rest and Digest." During this time, students are still and quiet. The lights are off as they grab a drink of water, make their way back to their seats, and settle in. I usually play soothing music or nature sounds from a YouTube video while students are making their way in, and all the while as their sitting at their seats. Using whatever materials students have easy access to, they can simply doodle. This is literally no prep on your part, reading while resting is an effective way for many students to calm down. Teaching children to control their breathing gives them the ability to focus, it also helps instill self-control, many deep breathing exercises can be explicitly taught, deep "belly breathing" is a great place to start. If transitioning from RECESS back to the classroom is what you want to focus on, you can check out the companion to Rest and Digest, Rest and Reflect.