What is the most scenic day trip to Switzerland from Como (in the south)?

You could take the Gotthard Panorama Express northbound (you would start at Lugano, 33 minutes away from Como San Giovanni by train).

https://www.sbb.ch/en/leisure-holidays/travel-in-switzerland/panoramareisen/gotthard-panorama-express/timetable.html

There is one each day in each direction, starting I believe in April. This train uses the old Gotthard Pass route that goes mostly over the Alps rather than under them in a deep tunnel. There is still a tunnel, but it is not nearly as long as the Basistunnel. The scenery is quite nice, and then you get a boat ride on the lake.

Then return on the regular train that goes through the Gotthard Basistunnel. There is one direct train a day between Como S. Giovanni and Luzern, but it runs at mid-day in both directions, so may not fit the GPE schedule. There are lots more options with a single change on the way (at Arth-Goldau, close to Luzern).

Or you could put together your own route over the Gotthard Pass route without using the GPE, but that involves quite a few transfers.

Do you mean you are staying in Como town? Or somewhere else on the lake?

There is no rail line along the western side of Lake Como, only buses or ferries.
As this is only a day trip, Luzern or the Gotthard route are way too far.

From Como town, I suggest Bellinzona, on the rail line north from Como town. Three castles (UNESCO World heritage site). Possibly combined with Lugano, which is on the same rail line between Como town and Bellinzona.

It sounds to me like they want to cross the Alps and go further into Switzerland. The same rail line that serves Como (town) and Bellinzona continues to the north side of the Alps (via the tunnel unless one take the alternate route using the Gotthard Panorama Express). It does make for a longer ride, but if they are looking for something similar to the Bernina Pass route, this would be it (and they get the bonus of a boat ride).

For something shorter, Bellinzona would indeed be a good choice. But it may feel like they haven’t left Italy, even if it is Swiss.

From wherever you are on the western side of the lake you might be able to take a boat to Varenna and then walk up the hill to the little station and take the northbound train to the end of the line. Change there for a train on the Bernina line which is also used by the Bernina Express. In addition to the fancy named trains there is a normal local one every hour. Take it a couple of hours to a place for lunch, have lunch, get on a southbound and go back the way you came.

You might like it. The first bit is in Italy, including an open-air spiral made by the track going over itself to climb steeply, other bits where the train goes right down the middle or side of the road, and then you climb higher and higher. Maybe Alp Grüm which is just beyond the Italian language part might be a good place for a light meal.

Have a look at https://en.wikipedia.org/wiki/Bernina_railway and https://en.wikipedia.org/wiki/Bernina_Express and https://en.wikipedia.org/wiki/AlpGr%C3%BCm(RhaetianRailwaystation)

NIgel, they were considering the Bernina Express route and specifically Alp Grum, looking at buying tickets.

https://community.ricksteves.com/travel-forum/italy/day-trip-lake-como-from-varenna-by-train-to-switzerland

But then they must have realized they are staying in Como, not Varenna. I think that is why they specified Como “in the south”. It is not quick or easy to get to Varenna (or Tirano) from there.

Thanks for all the responses!

Yes, we are staying in the town of Como. For some reason I originally thought we were staying in Varenna (not really knowing the difference when we booked the package). We were very excited about going through the Alps and having lunch at Alp Grum :(

So, yes we will be in the town of Como, and I understand it is at least an hour by ferry to Varenna, and 2 train rides after that. Researching I was finding Bellinzona or Lugano as the most logical options from Como but was hoping for some secret option number 3!

Thanks to all for responding!

Not a truly Alpine experience but Bellinzona is well worth a visit. It has a small, attractive old town and is a UNESCO site for its three castles/ruins. A fairly steepish but pleasant walk up to two of the castles, many of the houses growing vines in their back gardens. I've tried searching where we ate without much luck (Cantinin dal Gatt, I think). It was Italian, I know that, very popular and the food and wine was nice and suprisingly reasonable for Switzerland. It was also a market day (Saturday?)

Agree with Lugano option, not really far away from Como itself which will make a great day trip for you.

I would also highly recommend Lugano, as it's very accessible from Como. There are direct trains from Como S. Giovanni to Lugano with a travel time of 31 minutes. It doesn't get much easier than that!

Lugano is a very scenic and somewhat posh city right on the lake and there are some good sightseeing options there. For example, you could take the lifts to either Monte Brè or Monte San Salvatore for some great views. As I recall there were boat tours of the lake as well. Check the RS guidebook for other suggestions.