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For a view of both lakes without a long mountain walk, take the funicular to Harder Kulm. From the upper station, walk to the Two-Lakes Bridge: Lake Thun stretches one way, Lake Brienz the other, with Interlaken below. Leave time to look around before heading back into town.
Check the webcam before buying a ticket if you are going for the view. Clouds can hide much of it. The railway runs seasonally, and queues add to the outing, so check the last descent too. Bring a jacket. The walk up from Interlaken is a steep mountain hike.