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Spiezer Alpine Weinkultur offers tastings of wine grown above Spiez bay. Book a public tasting date or arrange a hosted visit, then walk through the vineyards while you are there. The sloping paths look down towards the castle and lake; you can also do the walk without a tasting.
Arrange the meeting place with the host; it is not necessarily at the castle. Wear shoes for the sloping paths, and leave the car behind if you will be tasting.