Mudlarking
While I lived in London I was an active mudlark. This was convenient when I worked in Soho and would walk back across Waterloo bridge, and could check the tide on a whim.
Some of my best finds have been garnets, a pattern shoe, a fourteenth century jug, many pins, and a piece of tracery from Lambeth palace. As a child I would often do long field-walks with my father to look for roman tesserae — mudlarking is a continuation of this habit.