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Oberhofen's castle stands right beside Lake Thun, with the boat landing close by. Come for the setting, then decide whether to visit the rooms, walk in the park or stop for a meal. A small village outing does not need to include every part of the museum.
Check the park and museum separately: they have different opening arrangements. The official visitor page is especially useful outside summer. The park is not a swimming or picnic lawn. For the return, a bus to Thun may suit you better than waiting for another boat; look at both options before settling into a long lunch.