The first of the Four Foundations Top Ten is, "I make it a habit to plan unit and daily lesson learning targets before planning activities."
Planning the targets first helps to focus our attention and time on what is truly important for our students to learn in that unit and that lesson. It also encourages us to keep the lessons tuned into the learning rather than the activity itself--learning activities are simply means by which students gain the target. With the target in mind we're in a better position to select the best activity.
Great teachers make it a habit to think through carefully the learning targets for each lesson prior to planning activities.
For more on this, see pages 5-7 of Four Foundations of Great Teaching