I spent 6 nights in the Lauterbrunnen/Murren area, plus 4 in nearby Thun (visiting a friend), and would have gladly spent more time in the area. There are a lot of great hikes and scenic rides in the area.
As others have suggested, it would help to know where you’re coming from and what your interests are. Also search some of the previous trip reports and threads to get ideas.
Depending on your interests, spending all or most of your time in the Bernese-Oberlander area is a good choice. If you want to check out cities, Interlaken is about a 30 minute train ride from Lauterbrunnen; Thun is an hour, and Bern is 90 minutes. So you can easily do day trips to one or more of these on rainy/cloudy days, then enjoy the Alps on sunny/partly cloudy days.
All the best,
Raymond