Hi All!
Heading to Frankfurt next month (Feb) for work and going to stop in Milan on my way. I have time for a day tour leaving Milan...any suggestions on which tours and what to see?
Any advice is greatly appreciated!

Moved to the Europe forum and tagged for Italy

I wouldn't take a tour outside of Milan. I'd spend the day IN Milan.

The Last Supper
The Duomo (including the roof)
Sforza Castle (including the walls)
Wander the Brera and/or the Quadrilatero d'Oro districts
Walk through the Galleria
Visit a museum (da Vinci, La Scala, Pinacoteca, Novecento, etc.)
Eat amazing food

I agree - why leave Milan?

Lake Como? Switzerland tour or stay in Milan?

Lake Como is close enough, but it would be cold and possibly rainy. Unless it was a private tour taking you directly to the places (like villas, mostly on the western shore) that you'd like to visit, it would be difficult to see much using ferries which would be on reduced, off-season schedules. In the tourist-oriented towns like Bellagio, lots of shops and restaurants would be closed for the season.

I'm not sure what you mean by a "Switzerland tour." Not many destinations are day-trip distance from Milan. If you were willing to spend the day on trains, you could go to St. Moritz for lunch and a short wander. Closer would be Lugano, but IMO it lends itself better to sunny days and exploring around the lake. Like Como, unless you're on a private tour, you'd have to investigate the frequency of the winter ferry schedules.