Florence - escape the crowds by going across the river on the Oltrano neighborhood; the Leather School near Santa Croce and the Santa Maria Novella Farmacia were fun visits. The latter was the prettiest retail store I’ve seen at the time.
Rome - I’ve read that the farmer’s market at Campo de Fiori is expensive, but I think it’s nice to walk through; if you want a food tour, how about one in a real market listed in this site - http://www.mercatidiroma.com/?
Siena - highly recommend staying within the city walls and walk the city at night. It’s surreal.
Venice - take in the views from the top of the Accademia Bridge at dusk; the Rialto Fish Market was excellent to watch and made me wish My hotel room had a kitchen to take advantage of what was being sold.