Lake Como is the reason many people add a day to Milan. It lies in Lombardy, in the provinces of Como and Lecco, and it has pulled visitors since the early 20th century on the strength of air, water, mountains and weather. The shape explains the planning: an upside-down Y, one leg from Como in the southwest, the other from Lecco in the southeast, joining halfway up before the water runs north to Varenna and Colico.
The southern ends are flat. Head north and the shore turns into Alpine foothill, with skiing in the valleys in winter. Boats run the length of the lake, but the distances are longer than they look on a map: two or three villages is a full day, not five.
Trains leave Milan for Como, which has two stations, Como San Giovanni and Como Lago, and you can buy a ticket at the station tobacconist for a few euros. Fast direct trains reach Varenna in about an hour, which puts you in the middle of the lake without a boat first.
Como
Como sits at the end of the southwestern leg and has been a silk town for a very long time, with the Silk Museum in the center and the Civic Museum Setificio Monti up the shore at Abbadia Lariana. The ferry piers are here, so treat the town as a morning and the water as the day.

Bellagio
Bellagio stands on the point where the two legs meet, which is why almost every boat converges on it. The regular ferry from Como takes around two hours and the hydrofoil about 45 minutes. The town is small enough to cover between boats, and it has the densest run of bars and cafes on the lake.
Varenna
Varenna sits on the eastern shore and is the easiest town to reach without a boat, because the direct train stops there. Villa Monastero is the sight in the village, and the pier puts you on the mid-lake triangle with Bellagio and Menaggio.
Menaggio
Menaggio looks across the water at Bellagio from the western shore, and the road in from Lugano lands here. Crossings inside the triangle are frequent, and Villa Carlotta at Tremezzo, with its gardens, is the next stop along the same shore.
Buy the day pass that covers most of the lake, and budget for the hydrofoil supplement on top. The western shore road is slow, so let the boat do the work.