A cancelled meeting leaves an hour free. You spend the first part wondering which of several overdue tasks deserves it, then begin something too large to finish before the next appointment.
Keep a few tasks marked with the kind of window they need. One may suit a short admin gap; another may need quiet concentration. Include something restorative if the day is already demanding.
Check the conditions
Before choosing, look at where you are, what tools you have and when you must stop. A free hour in a noisy lobby is different from one at your desk. Pick a task that can reach a useful stopping point in those conditions.
If none fits, use the gap for preparation or a break. You do not have to turn every cancellation into extra output. Afterwards, note whether the choice worked. A small set of ready options can save you from repeatedly deciding from your entire task list when you have only a modest stretch of time available.
