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The water draws attention, but leave time for the villages too. Brienz has a promenade, woodcarving heritage and a railway station, making it worth considering without a car. Smaller places such as Iseltwald have different connections; check the bus or boat from the actual address, along with food shopping and dinner.
Lake Brienz boats use Interlaken Ost, not the Lake Thun departure at West. Check seasonal sailings and the whole return connection. You can visit both lakes without moving accommodation. If waking beside Lake Brienz matters most, choose a stay there: the homes listed on this site are around Lake Thun.