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Faulensee suits an afternoon by the water: a shoreline walk, something to eat and no pressure to add another excursion. The path towards Spiez gives the visit a natural direction. If you want to return by boat, check the actual sailing first; keep the bus as another option.
For an overnight stay, look beyond the lake-view photograph. Check the walk to the bus stop, groceries and restaurants. Buses connect with Spiez station for onward trains, so an early mountain departure includes that first transfer. A small village can be peaceful without every road or apartment being silent.