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Castles and villages look different from the water, and you need not cross the whole lake to enjoy them. A section of a scheduled boat route leaves time for lunch ashore. Check the seasonal timetable and how you will get home before choosing a landing stage.
For something hands-on, Segelschule Thunersee offers introductory sailing around Spiez and Hilterfingen. A yacht trip with a skipper is a separate enquiry. Confirm the meeting place, actual sailing date and weather arrangements with the school; a winter theory course is not a winter outing on the lake.