6 days in Northern Italy...

No Venice or Verona? Milan is worth a full day, imo, not more. My ranking of favorite cities in North Italy: 1. Venice 2. Verona 3. Many others more or less equally ranked. To name some of the better 3rd bronze medalists: Bologna, Parma, Ferrara, Ravenna, Milano, Padua, Vicenza. Lake Como (from Milan): go to Varenna (maybe ferry from here to Bellagio)
Lake Garda (from Verona): Sirmione, Salo', Limone, Riva

Half a day doesn't seem long enough for Lake Como, especially if you want to ride the slow ferry up the lake to enjoy the views. I'd consider allowing a full day for the trip. You can start at either Como city or Varenna (I'd probably choose Como) and work your way along to the other, returning to Milan by train. There are interesting places to see in several towns along the way and the Isola Comacina is definitely worth a stop to see the ruins if you are interested history.

Alyssa, I'm also not sure that half a day is adequate to visit Lago di Como. As others have suggested, you could either travel to Varenna by train and then visit Bellagio via boat (about 20 minutes each way), or travel up the lake from Como and then return from Varenna by train. I agree with Roberto that Verona would be a great option to consider. Even if you're not interested in the fictitious Romeo & Juliet site, it's still a beautiful city with an impressive Roman Amphitheatre. Another possibility would be Stresa, which is about an hour from Milan. While there you could tour a few of the Borromeo Islands and especially the elegant Villa and gardens of Isola Bella. For something a bit different, you could also take a day trip to Locarno (CH), which is only about two hours from Milan (depending on which train you choose). It's a beautiful (and somewhat "posh") city on Lago Maggiore, surrounded by mountains. While there you could look through the shops, visit the Casino, have a look at the impressive Madonna del Sasso Franciscan Monastery or take a Cable Car ride to Cardada, which provides awesome views of the city and lake. The last train back to Milano departs at 20:45 AFAIK. Happy travels!

I advise you to visit Mantua, beautiful city and great food and maybe even Bergamo, 30 miles from Milan, worth a visit.

We went to northern Italy in October and Verona was our favorite place. Partly because of a resaurant we found and partly because of the Arena built by Augustus Caesar.

Thank you all for the honest insight! We hadn't considered Verona, but I will make a point read up on it. I definitely will ensure we spend more than half a day in the Lake Como area. My husband didn't want to dedicate too much time there--we have vacationed previously on the Amalfi coast and got bored very fast in Sorrento and Positano so we didn't want to make the same mistake twice. Any other thoughts appreciated!

On the Lake Como idea. Over the years, we have visited the lakes region several times (different lakes each time); that part of Italy just didn't "click" with us, and we found it relatively boring. Lucky for all of us, not everybody likes the same things. We also did not find Sorrento interesting, though the views from our hotel rooms were fantastic. Use your prior experiences to determine what you would enjoy the most. After years of skipping Milan, we visited there several years ago, when my husband had some business meetings there. I was happily surprised at how much I liked the city. One of the things I liked, though, was the number of smaller museums that we visited. So if you're not big art and church people, you probably won't have places like that on your list of things to do in Milan. I LOVE Bologna. It's a manageable size, great for walking around, a number of interesting places to visit (including their art museum), great markets and people-watching.

Lexma: Thanks for the honest insight and nice to see there are more people like us out there :0.

If you love food..... take a cooking class (it is a great experience!). In particular my husband and I took a cooking class about the art of pasta making in the Veneto region, in Padova (a train ride: 2 hours) and it was a fantastic thing to do. Take a look at http://isacookinpadua.altervista.org