Weather forecasts are notoriously unreliable, especially in mountain areas.
They really are only good for a few days in advance.
Also, if there is rain expected anywhere in an area, it has to be shown in a forecast. The weather in Switzerland varies quite a lot over small areas, due to the geography. So, often I go out on a day with a forecast for rain and find only clouds or even sunshine in the small area where I am going. Not always, of course.
The Meteocentrale weather site has a good map showing localized weather details in Switzerland. I don't know the site well, so I don't know about its forecasts, but they have a good reputation:
If their forecasts , or any forecasts, show sun peeking out from the clouds, there is at least some hope for that day in that region.
Also, the Rhone Valley often is dry, and even sunny, when the rest of Switzerland is wet.
A train ride via Kandersteg, to Brig might prove to be clear enough, especially on the southern end, to be enjoyable. Go over the top via Kandersteg, on the local train "Lötschberger" not through the new lower fast and boring "basis tunnel."
Bern is a town whose appeal is not completely eliminated by bad weather:
Those thoughts cannot withstand a forecast that is, literally,
<<"weather is looking mean everywhere for next 8 days.">>
It can certainly be disappointing to go high up in the Alps and be inside a cloud.
Sometimes, there is a cloud deck, and the weather is tolerable, if not bright and sunny, beneath it.
So, for example:
Also, for what it is worth, the forecast for 21.6 and 22.6 in Interlaken looks to be clear and sunny. Not that I trust it very much.
The Ticino -
This region south of the Alps often has weather completely different from the rest of Switzerlnd,because it is south of the Alps. You thus might consider a train ride to Locarno or Lugano. Both are attractive towns; Locarno is a bit more compact.