Don't let the weather in Murren worry you too early. We arrived here 4 days ago when rain was predicted. We have had sunny, hot weather. The forecast that I saw here tonight was sunny starting next Monday.
The locals say to check only the local weather forecast for better accuracy.
Rick Steves has info in his Switzerland book on what to do for rainy days.
Bring your bathing suit as the hotels here give you free passes to the amazing Sports Complex. We used the gorgeous inside pool. Check with your hotel. There are programs such as yoga at the complex too.
Have a great trip, it's an amazing area to explore!