Lahti sits at the southern tip of Vesijärvi, a long narrow lake stretching twenty-five kilometres north into the Päijänne lake district. The harbour at the foot of the city — a pleasant marina with restaurants and a small fairground in summer — is the start of a paved cycle and walking path that runs along the eastern shore.
The lake is good for swimming from June to early September, and you can hire kayaks, rowing boats and small electric boats at the marina. In winter the lake freezes hard enough for ice fishing, ice skating and the odd cross-country ski track.