As a couple, Rialto Bridge works.
Crossing the Rialto once is a genuine rite of passage and the view straight down the Grand Canal from the crown of the arch is the real postcard. But the bridge is the least interesting thing about Rialto: the shops on it are tourist-tat jewellery and souvenirs, and you'll spend longer queuing to photograph it than enjoying it. The actual reward is at its western foot — the Rialto Market at dawn and the cluster of true bacari (Do Mori, All'Arco) where Venice still eats standing up. Treat the bridge as a 90-second crossing on the way to the better thing.