Lake Thun stretches 18 kilometres from the town towards Interlaken, hemmed in by mountains on both sides. The water is a striking turquoise-blue — the colour comes from fine glacial silt suspended in the water — and the mountain backdrop, with the Bernese Alps rising sharply from the south shore, makes it one of the most dramatic lake settings in Switzerland.
Paddle steamer services run the length of the lake in summer, calling at Spiez, Faulensee, and Interlaken. The voyage from Thun to Interlaken takes about two and a half hours — a leisurely way to travel compared to the train, and worth doing at least once. Swimming beaches are at several points around the lake; the public beach at the southern end of Thun, near Schadau, is the most convenient. Sailing, kayaking, and stand-up paddleboarding are all available in summer.