As a couple, Ponte Santa Trinita (sunset spot) works.
This is where the postcard shot of Ponte Vecchio glowing over the Arno is actually taken, and at sunset the light on the river is the city's most romantic free moment. The catch: it's no secret — the parapets fill with photographers at golden hour — and there's nothing to 'do' beyond stand and look. A five-minute payoff that punches far above its effort.