Punkaharju is some 35 kilometres from Savonlinna, built around one of the most distinctive landscapes in Finland: a long, narrow esker — the Punkaharju Ridge — that snakes through the lakes for several kilometres with water on both sides. Pines grow straight out of the gravel, the old road runs along the crest, and the whole thing has been protected since 1843.
The town itself is small, a few thousand people, and there is no real centre to speak of. People come here for the ridge and for Lusto, Finland's Forest Museum, which covers the country's deep relationship with its forests far better than the dry topic might suggest. Add the nearby Punkaharju Arboretum and the National Landscape viewpoints along Highway 14 and you have a half-day or a full day, depending on how slowly you take the walks.
It is best combined with a longer trip to Savonlinna and the Saimaa lake region. Coming all the way out from Helsinki for Punkaharju alone is overkill; folding it into a wider Lakeland itinerary is exactly right.