San Frediano is the Oltrarno with the polish left off — the working borgo behind the old city gate, where the great Masaccio frescoes sit a few doors from the bars that genuinely belong to the neighbourhood. Do it in the afternoon, so the art comes first and the street comes alive after.
Under three kilometres. Start at Porta San Frediano, the tall 14th-century gate with its original doors still hanging and its tower famously never finished. Walk the length of Borgo San Frediano, past bottegas and bakeries, to Santa Maria del Carmine. The brick facade is a barn; inside is the Brancacci Chapel, where Masaccio painted the Tribute Money and the Expulsion in the 1420s and reinvented how a painted figure could occupy space and feel pain. Book ahead — entry is capped.
Sit a while in Piazza del Carmine to let it land, then thread the back lanes around Via dell'Orto down to the water. End at Ponte alla Carraia, with the Lungarno running east toward Ponte Vecchio and the evening just starting behind you.