Everyone photographs the Arno at the Ponte Vecchio and nobody walks it downstream, which is exactly the mistake to exploit. East of the centre the river widens, the crowds fall away, and you get the embankment, the weir and the medieval towers more or less to yourself — best of all in the last hour of light.
Start on the Lungarno della Zecca Vecchia, named for the mint that struck the gold florin, and walk downstream with the water on your right. The set piece is the Pescaia di San Niccolò, a low weir where the Arno goes white and loud — lean on the parapet and listen. Past the lone Torre della Zecca Vecchia, cross at Ponte San Niccolò to the south bank and the San Niccolò quarter: one long street under the hill, the Porta San Miniato gate, bars filling at dusk.
Stand under the Torre di San Niccolò — the one gate-tower the 1860s demolitions left at full height — as the light goes. Then back upstream along Lungarno Serristori, the whole centre catching gold across the water. End at Ponte alle Grazie; cross on foot, or take the bus from the south bank.