The cathedral is the obvious centre of gravity, and any visit to Canterbury starts there — allow at least half a day. Beyond it, the city is small enough to walk in a morning. The medieval centre still sits inside its Roman and Norman walls, with three UNESCO-listed sites (the cathedral, St Augustine's Abbey, and St Martin's Church) all within easy reach of each other.
The non-religious highlights cluster too: the Roman museum sits over an in-situ Roman mosaic; the Beaney is a free, oddly likeable Victorian museum-and-library; and the city's medieval gates and walls give you the surviving outline of a fortified pilgrim town. The river is part of the appeal — book a punt for a half-hour drift past the old city walls and the gardens of the King's School.