Another vote for Treviso. Other very nice places on the way from Venice to Milan are Padua, Vicenza and Brescia. All very attractive. Vicenza is quieter than the other two. There's also Verona, lovely but probably too touristy and busy for you.
Note that Padua, Vicenza, Brescia and Verona can all be really unpleasantly hot at the very beginning of September. The temperatures seem to drop considerably as you move through the month. Over the last five year Treviso hasn't suffered from oppressive heat at the beginning of September, but from recent personal experience I will say mid-summer there can be awful.
All the places I've mentioned, including Treviso, are known to tourists, just not necessarily to a lot of foreign tourists, and they are not tiny. I'd recommend Googling for population figures and looking at Google Maps to get a sense of the size of the recommended places. A lot of truly tiny places will be quite time-consuming to reach.