Lebell's Merchant House is the best indoor stop in Kristinestad, a preserved bourgeois home from the period when the town's merchants made their money from shipping and coastal trade. The rooms show furniture, domestic objects and interiors from the late 18th and 19th centuries.
It is a small museum, but a useful one: after walking the wooden streets, you get to see what prosperity looked like behind the facades. Summer hours are the most reliable.