Day-trip from Maggiore into Switzerland

From Stresa, I would take the train north to Brig (in Switzerland) and change there for a train going east to one of the base stations for the alpine villages high above the Rhône valley. It is only a 10-15 minute journey from Brig, and then a cable care takes you up 1000 meters or so to a car-free village high above. From there you can ride lifts up to the ridge to see the huge Aletschgletscher, Europe’s longest glacier. And stroll the town, have lunch, or take a short hike.

We always go to Bettmeralp (valley station on the train line is Betten), but Riederalp would work too.

https://www.aletscharena.ch/nature-en/the-great-aletsch-glacier/

Or go a bit past Betten to Fiesch, and ride up to Fiescheralp and on to the top of the Eggishorn, from where you can see up the glacier all the way to the Jungfrau.

https://www.aletscharena.ch/nature-en/the-great-aletsch-glacier/

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You might enjoy taking a day trip to Locarno (CH) via the scenic Cento Valli railway. Here's one route you can consider.....

Take the regular train from Stresa to Domodossola and once there transfer to the Cento Valli. The station is undeground and separate from the Domodossola station. It's a very pleasant trip and the scenery is great.

After arrival in Locarno, there are several interesting sights.....

• the Madonna del Sasso Franciscan Monastery (above the town - as I recall, there's a funicular that will get you there)
• take the cable car to Cardada / Cimetta as the views are spectacular - https://www.cardada.ch/en . The cable car is a bit pricey but that's normal for Switzerland. As I recall, there's a hotel & restaurant right next to the top cable car station. They have a large outdoor patio so you can have a cool one or a fine meal and enjoy the views, while the paragliders sail past.
• if you want to try your luck, there's a Casino
• the walk along the waterfront is a nice way to enjoy the sights.
• take a walk around Piazza Grande as there are some nice restaurants and stores. You could always stop at Bucherer jewelry and buy that CHF 15,000 Rolex watch you've always wanted.

For the return to Stresa, use the Ferry or Hydrofoil. If you depart Locarno in the late afternoon, you'll be back in Stresa in the early evening (I haven't checked the current schedules).

As you'll be in a different country, don't forget to take your Passports.