Rättviks Gammelgård is the town's open-air homestead museum, a collection of older buildings and local objects that explain the rural culture around Siljan. It is a small, gentle museum rather than a headline sight, but it helps put the red houses, folk costumes, and craft traditions into context.
Pair it with the church peninsula rather than treating it as a separate excursion. Together they make a good short walk through Rättvik's older side before you return to the lakefront.