Activity definition is a key step in project plan development. After developing the work breakdown structure as part of scope definition, verify that all work listed is necessary. If the work at the lowest level will probably require more than a month to complete or consume more than 80 hours of effort, strive to break it down further.
Work related to organizational, business, or legal requirements is often overlooked. Examples include preparation for life-cycle checkpoints, methodology requirements, project and other reviews, scheduled presentations, and specific documents the project must create. Add any missing work you discover to the plan.