Sipoonkorpi National Park is one of the easiest ways to get real southern Finnish forest close to Vantaa and Helsinki. The park protects spruce woods, rocky hills, small fields, and seasonal berry and mushroom grounds.
It is a strong half-day nature trip if you want something wilder than a city park without travelling deep into Finland. Trails can be wet and rooty, so sturdy shoes help, and in summer the forest feels much farther from the airport suburbs than it really is.