We made good use of the Millenium Bridge, built over the Thames in 2000. It’s a pedestrian-only bridge, and after the initial opening it was closed for 2 years to fix a “wobble”. We’re happy to report it was nice and stable when we crossed.
On the north side of the bridge, there is footpath along the northern edge of the Thames just south of St. Paul’s.
Nearby, when the tide is low, hardy beachcombers venture onto the temporary beaches at the edge of the water. Some carry metal detectors.
Due to the long history of London and the Thames being used for transport, historical and valuable artifacts often pop to the surface. The shores in front of the Tower of London turn up the most finds, but they are only opened to the public on rare occasions.