Kuusijärvi is where Vantaa shows its Finnish side most clearly: a lake, forest paths, public saunas, and people treating cold water as a perfectly reasonable idea. In summer it is an easy swimming and picnic spot; in winter the smoke sauna and ice swimming make it much more memorable.
The recreation ground also works as a gateway toward Sipoonkorpi National Park, so you can turn a short sauna visit into a longer nature day. Bring swimwear, a towel, and a flexible attitude toward weather; this is a local routine as much as a tourist attraction.