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Thun deserves a visit even without a mountain excursion. From the station, head towards the Aare and old town. On Obere Hauptgasse, look at the raised pavements: the shops occupy more than one street level. Mühleplatz is a natural place to stop for something to eat.
The old-town riverfront and the lakefront are different parts of the visit. Continue towards Schadau Park when you want the open water. For a stay, Thun offers town services and restaurants, with a longer journey into the Jungfrau valleys than Interlaken. Choose it because you want time in Thun itself.