Hello, we are trying to decide whether to venture into the Berner Oberland region knowing it's going to rain the entire time we are there. I have decided to skip Jungfrau because of the price. But should I even try to go to say Grindelwald, etc? How about Trummelbach falls and/or Aare Gorge? Our time is limited to one day, but we could spend the night nearby (or just stay in Montreux and do the chocolate train)?

"knowing it's going to rain the entire time we are there" which day? today?

Go visit Bern if it is going to rain all day in the mountains.

Trummelbach falls in Lauterbrunnen are spectacular when it rains because you are inside the mountain - very unique. Sure check the weather forecast for Jungfrau, but Gridlewald glaciers are also stunning in the wet: www.gletscherschlucht.com and so is nearby http://www.aareschlucht.ch/ and also the amazing https://www.tropenhaus-frutigen.ch/ is something different.

Places to visit in the Berner Oberland if it rains:
St. Beatus Caves
Trummelbach Falls
Oberhofen Castle
Tropenhaus Frutigen (as above)
Wocher Panorama Thun Schadau
any museum at Berne
any covered swimming pool or fitness center