We have no interest in driving into Milan but would like to go for a day in the middle of a trip in which we already have a car. Can anyone recommend a place where we would have hotel parking and be able to take public transportation into the city? North side of town would be preferable as we would be going to Lake Como the next day. Thank you for any suggestions.

Near Malpensa airport would be one place.

Trenord - Regional & Suburban Railway

Tren Nord runs lots of commuter rail lines - study the maps and find a hotel on northside in town or area north of Milan.And from Lake Como Milan is a short rail ride so consider doing away with one-night stand and day trip to Milan. Even from Bellagio it is easy to take ferry to nearby train station at Varenna.

I'd also first check Pal's suggestion to see if your place at Lake Como was close to any railway station.
For example, from Como proper, the fastest regional trains take you into Milan in just a bit more than half an hour.