Standard 1 - Starting Lessons

The first Standard of Great Teaching at Markham College is:

This is an activity designed to maximise learning time for students by getting them working and thinking as soon as they enter the classroom.

These are not prescriptive in nature. There are many different activities that make excellent Do Now activities, and the nature of the activity will depend on both the subject being taught and the class. But they should involve all students working as soon as class begins.