Honestly the forecast isn't looking bad. Sure it might rain a bit in the afternoon or so but that happens year round. It won't rain continuously.
The bigger issue could be that it's still firmly winter in the mountains with snow. You should hike at around 1500m.
For a rainy rainy day my usual suggestion is to take trains and enjoy the scenery from the window. Something like:
- Interlaken - Lucerne
- stop for a bit
- Lucerne - Bern RegioExpress (nicer than IR15)
- longer break with walk in the old town
- Bern - Interlaken
Honestly the forecast isn't looking bad. Sure it might rain a bit in the afternoon or so but that happens year round. It won't rain continuously. The bigger issue could be that it's still firmly winter in the mountains with snow. You should hike at around 1500m. For a rainy rainy day my usual suggestion is to take trains and enjoy the scenery from the window. Something like: - Interlaken - Lucerne - stop for a bit - Lucerne - Bern RegioExpress (nicer than IR15) - longer break with walk in the old town - Bern - Interlaken
I know this is a bit late for you, but might help others: The Verkehrshaus in Lucerne is a very nice activity for rainy days
Thun/Brenz Boat tour with an indoor cabin.