Dolomites or Switerland from Lake Como?
 

Next fall (Sept 2024) DH and I will be in Lake Como for a week after spending a couple days in Milan first. Thinking that I'd like to fit in a few more days before heading back home I started thinking that we could maybe fit in either a trip up to Switzerland or the Dolomites for a few days. We won't have a car, so wondering if heading back to Milan from Como then catching a train up to Bolzano in the Dolomites would work. I understand that there are hotels not too far from their train station and that there's a lot to explore in Bolzano and also nearby hiking trails. I've also read about trips from Como up to Switzerland. Closer to Como, but not sure where to go/stay. Just wondering if anyone has any thoughts or ideas on either of these excursions. Looking at spending just 2 or 3 nights, so don't want to travel too terribly far. Thanks!

From Lake Como the logical (most convenient) choice would be the Upper Engadine in Switzerland. The surroundings of St. Moritz (stay in a village nearby, e.g. Sils-Maria, Pontresina.) To get there you take trains to Chiavenna, from there the Swiss Postal bus, or you take the Palm Express bus from Menaggio (no changes.) Short and scenic ride.

Frankly, Bolzano is not really *in* the Dolomites. It is a larger town (small city), bustling, with a quite beautiful old town but not so nice outskirts, and while it is a good hub for excursions to the Dolomites it requires quite some time to get up there.

For only 2 days, Bolzano/Bozen (110 000 inhabitants) is too far away.
The closest places are the Upper Engadine (recommended by Ingo) and the Upper Valtellina (Bormio), both about 3 hrs by train and bus from Varenna
The Engadin is much nicer.

Take good look at charming Pontresina. We traveled to Lake Como from Pontresina.

True that the Engadine is more convenient to Lake Como and is very beautiful. But we've been to St. Moritz and Davos, and didn't find anything special in the towns themselves. The best part of this area is the Bernina Express train excursion and the countryside itself.

Bolzano/Bolzen is a comfortable mid sized city. It's attractions are shopping, Otzi the ice man museum, and the nearby Val di Gardena and Alpe di Siuse. Unfortunately, you would have to rent a car in Bolzano or navigate the regional bus system. Buses provide good coverage but there's a leaning curve. Also, as pointed out, it's more difficult to get to Bolzano from Como.

Is it doable to go to Pontresina from Lake Como as a day trip?

Varenna - Pontresina by public transport takes 3 1/2 hrs, either via Bernina Pass (all by train) or via Maloja Pass (train and bus). Departures every 1-2 hrs from 7.25 until 17.25
If you want to go into the glacier areas, leave the train or bus before arriving at Pontresina and go by gondola to Diavolezza or Piz Corvatsch