A coastal town is a good place to do very little in, and Poole is better at it than its summer reputation suggests.
Early: the water
A dawn swim or a flat-water paddle before the harbour wakes up. In season, swim on the lifeguarded beaches between the flags; outside it, swim with others and read beach safety. See watersports.
Morning: a walk with nothing to decide
The harbour shoreline is flat and continuous for miles, which is exactly what you want when you do not want to think about navigation. See walking and trails.
Midday: a garden
Compton Acres has a tea room among the planting, which turns an hour into an afternoon.
Afternoon: a spa, or a long tea
Hotel spas around the peninsula and Canford Cliffs run day packages. Or just book afternoon tea somewhere with a view and stay there.
The quiet places
Upton Country Park, the Hamworthy shore and the heath at Arne are quiet even in August.
See health and wellbeing.