Postman's Park is a small City of London garden with a memorable reason to stop: the Watts Memorial to Heroic Self-Sacrifice, a tiled wall recording ordinary people who died while saving others. The park combines the old churchyards of St Botolph's Aldersgate, Christ Church Greyfriars, and St Leonard, Foster Lane, so it feels more layered than a simple patch of grass.
It is close to St Paul's Cathedral and works best as a quiet detour while walking through the City.