Haverfordwest Priory

Five minutes' walk from Haverfordwest's shops, on the flat ground by the Western Cleddau, sits something most of the town's own visitors never find: the ruins of a 13th-century Augustinian priory, excavated over years of careful archaeology and now in Cadw's care, free and open access. The excavations revealed the layout of the priory's medieval gardens, which have been replanted on the evidence — raised beds where canons grew their herbs seven centuries ago — a rarity anywhere in Britain.

It's a calm, unstaffed, riverbank sort of site: swans, ruins, interpretation panels, and rarely another soul. Perfect paired with the riverside walk into town, and the free counterpart to Haverfordwest Castle on the crag above while that emerges from its regeneration.

Free, open daylight hours — details on Cadw's page. More like it in the free things to do guide.