I would hit the Duomo, the galleria and not much else. Hike to the top of the Duomo, wander the interior and eat pizza in the piazza. To get the full effect take the subway to the Duomo stop and walk up the stairs. Directly across from the Duomo the walk out of the subway is impressive.
In order of interest 1. Duomo inside and hike to roof (1-2 hours). Sadly the outside is encased in scaffolding during 2006. 2. Duomo museum (1 hour) 3. Galleria Vittorio Emanuele - for coffee and people watching (as long as you remain interested. All are right on the Piazza Duomo. 4. after a short walk you could also hit the Piazza del Scala (opera house) if you time it right you could attend an opera since the schedule is now nearly constant or if you are a buff you could visit the museum. None buffs would be sorely bored.
If you want to see the lakes you could also do that after seeing the Duomo/roof/galleria by taking the subway back to the train station and taking the 30-minute train ride to Como on Lake Como. Como is a walled city and both it and the Duomo are worth seeing. The Duomo is one of those enigmas a Gothic facade with a Renascence dome (actually 18th century I believe).
You can also take the funicular up to the little town of Brunate (so I have been told - we have never done this). We did take a funicular on the west side of the lake, with bikes one year and it was worth the effort.
Any way the train to and from Como should be quick and painless and easy.
I would dedicate 6 hours or more to seeing the Milan Duomo and area and then hitting Como and then returning to a central hotel. While taxis are a good idea on such short notice I would first stop by the train station and buy my tickets to and from Como and from there ride the subway to the Duomo and back, leaving only cabs to and from the train station. Cheaper and faster really.
I hope this helps and if my syntax is out of shape it's because we just returned from Italy. Ironic no?