Bernese Alps

I'm not sure that a day trip from Stresa to the Berner Oberland will be practical. For example, travel by train from Stresa to Lauterbrunnen ranges from about 2.5 hours to ~4 hours each way. I suggested Lauterbrunnen as you mentioned hiking, but there may be closer areas that would also be suitable. One other point to consider is that in April, there may be snow on the hiking trails. Hopefully one of the others will have a suggestion that will help you achieve your plans.

One other possibility you could consider that would be easier from Stresa, is to take the scenic Cento Valli raillway to Locarno (CH) and then return by boat in the evening. That trip is fairly easy as a day trip (I know as I've done it). In Locarno you could take the cable car up the mountain to Cardada / Cimetta - https://www.cardada.ch/en . In addition to the outstanding scenery, hiking should be possible there (again, depending on the snow situation).

If you want a day trip in Switzerland, Zermatt would be quicker to reach than the Berner Oberland. It's very doable. I would consider it, but check weather before you go. April is still snowy, so any hiking will be winter hiking, but the mountain views will be there, weather permitting!

I don’t actually mean the Berner Oberland - my understanding is that the northern part of the Bernese Alps is included in the BO, but not the southern part. It looks like one could take a Trenitalia train as far as Domodossola, then a Swiss train further north, which would connect with an east-west train that runs just south of the Bernese Alps. It stops at several small villages. I was wondering which of these would be a good place to get off the train and explore.

Any long day out, the train ride itself will be the main attraction. The stops between Domodossola and Interlaken, for example, are few and far between. Brig is probably the main stop in between, to catch an east-west ride. Montreux one way, Chur the other – been to neither, but the scenery will be splendid.

We based on Lakes Como and Orta. From the latter we popped over to Maggiore three times or so. A suggestion is to take the Centovalli from Locarno to Domodossola and back (bus runs on west side of Maggiore?). We took the ride in reverse. Domodossola may not be anyone's idea of a good detour but is ok for lunch and a short walkaround. One day in the car we stopped off at Santa Maria Maggiore, a neat little place.

https://www.vigezzinacentovalli.com/en/amazing-journey.html

Don't know about public transport times from Stresa but we visited Bellinzona from Lake Como. That was a real good day. Market day, I think a Saturday.

https://www.bellinzonaevalli.ch/en/

Thank you Gundersen - interesting suggestions. When looking at the map, I noticed that several of the small towns just south of the mountain range have gondolas, but am I correct in thinking that they most likely will be out of service in April?

It depends when in April. Early April is still winter season and gondolas should run. Late April, less likely!
Kandersteg would be an easily accessed location; you would need to check if Oeschinensee is accessible at your dates. You'd need snow-appropriate footwear though.

Thank you, Balso!