Bergamo is worth a day trip, check out both the upper and lower parts of the city, you can hike all the way up the highest part of the city (or take the two funiculare and then walk the final part,) which looks like something out of fairy tale. Citta Alta' is all medieval, while the lower city part of the city is a little more recent. Citta' bassa is also great for shopping, having an aperativo, and enjoying the evening passeggiata, and just checking out all the well dressed Italians. I think you should give yourself the day though.
Verona is also beautiful, I can't imagine trying to blow through it in half a day, especially at the train takes several hours to get there.
As for the area around Lake Como being cold, well, yes, it would be cold, but still very pretty. Unless you were planning on taking a ferry ride around the lake, you wouldn't be missing much as far as enjoying the lake. There aren't big town beaches or anything, and Lake Como is considered better for boating than actually swimming.
Other towns to consider are Brescia, Mantova, Parma, Salo' and if you really wanted to do a long day trip-Turin.