St Botolph's Aldersgate is a medieval City church rebuilt in the late 18th century, with a calm interior that rewards a short look if you are walking the lanes north of St Paul's. Its churchyard became part of Postman's Park in 1880, named for the postal workers from the nearby General Post Office.
Visit it as part of a City churches walk rather than as a stand-alone sight. The mix of church, park, and surrounding office streets is very much old London folded into the modern Square Mile.