Venice rewards even a single day, but only if you focus on the compact core around St. Mark's and skip the outer islands and side museums for another trip. This itinerary sticks to what's walkable in one day: the grand set pieces of St. Mark's Square and a wander through the quieter canals nearby.
Arrive early and head straight for St. Mark's Square before the tour groups fill it, taking in the Campanile di San Marco for a rooftop view over the lagoon. Step into St. Mark's Basilica to see its gold mosaics, then move next door to the Doge's Palace, crossing the covered Bridge of Sighs on your way through. By midday, walk the twenty minutes north to the Rialto Bridge, pausing at the market stalls along the Grand Canal for a quick lunch. In the afternoon, cross into the Dorsoduro District and spend an hour at the Peggy Guggenheim Collection for a change of pace from the Byzantine grandeur. As evening falls, make your way back toward St. Mark's Square, where the basilica and campanile are lit up and the crowds have thinned, and close the day with a coffee at one of the square's historic cafes.