Seurasaari is a beautiful open-air museum. The countryside is stunning - acres of forests and fields - and the buildings are fascinating. The museum contains buildings transported to Seurasaari from all over Finland, so you get to see examples of all sorts of early Finnish architecture, from farm houses to churches to manor houses, all filled with period furniture. The museum is easily accessible by bus from Helsinki, and admission is cheap, so there's no reason to miss it. Definitely try to go during the summer when the buildings are open and you can go in them.