I think over anything it is what interests you should drive where you go. Also, how you're getting home. If you are flying home from Milan, it makes sense to stay in the Milan area, for instance.
Venice, imho, is a treasure. So much to see - even so much away from San Marco (which is also worth seeing. Pre-book major sites like Doge's Palace, Basilica San Marco). Being in the city, especially if you're away from the crowds, can be enchanting. Murano, Burano and Torcello are also great to visit. For reference, I went to Venice twice in August/September. It was crowded. But you can manage it by planning around the crowds. Definitely pre-book most of the sites in San Marco if of interest. There is so much to see in the city AWAY from San Marco and those crowds that you can really take it all in.
Milan is often underrated. But the Duomo, The Last Supper (if you can get tickets or an independent tour) and Castello Sforzesco are worth visiting if of interest. Bergamo is nearby, as is Como. I have not yet been to Como, but judging by my knowledge and own research, it is less about sightseeing as it is for relaxation and recreation.
Florence is heavy on Renaissance art and architecture. If that is of interest. Yes, there are other things to do and see, it just is the focus of the city. It is recommended to prebook major museums (Uffizi, Galleria d'Accademia, Duomo dome, etc.).
Bologna is a nice mix of excellent food and sightsee.
Verona has a nice variety of interests with a Roman arena that is still used for concerts and performances, a castle, and many more. It gets a lot of day trip traffic, so staying overnight is worth it.
If you give us a better idea of what you're interested in and where you will be coming home from, we may be able to offer more specific suggestions.