Lahti Harbor (Lahden satama) is a summer paradise within walking distance from the city center of Lahti. The best walking route from the market square is along Vapaudenkatu Street and Kartanonkatu Street to the musical fountain and Pond Pikku-Vesijärvi and then walk along the lake shore. You see Kariniemi Arboretum and Kariniemi Fitness Stairs on the way before arriving to the harbor. The distance is about 1.5 kilometers (20 minutes). It is also possible to rent a city bike because there is a city bike station in the harbor, too. Also electric city bikes and electric scooters are available.
There are interesting sights in Lahti Harbor: Sibelius Hall (a concert house with interesting architecture and art) and Pro Puu Center (high quality Finnish wooden products to see and buy). Read separate Tripadvisor reviews about these sights.
There are nice cafeterias, bars and restaurants in Lahti Harbor. Some of them are restaurant boats and some of them are lakeside houses with good summer terraces. Lake view is fantastic. There is also a small playground for children next to Kahvila Kariranta (Cafe Kariranta).
The best part of the harbor is a possibility to take part in interesting cruises. M/S Kymppi has cruises to the beautiful Enonsaari Island (30 minutes in one way, a round-trip ticket 20 euros). Read separate Tripadvisor reviews about Enonsaari.
Päijänne Risteilyt Hilden is a cruising company which offers many different cruises. During the high season you can take part in daily lunch cruises (usually at 1 PM, 4 hours) and summer evening cruises (usually at 6 PM, 4 hours), not on Mondays and Sundays. Longer cruises go to Heinola (4½-5 hours in one way, two canals on the route, lunch buffet available), Jyväskylä (10 hours 30 minutes in one way) and Lahti (also 10 hours 30 minutes in one way). The cruise Lahti-Heinola-Lahti is possible during one day but it is also possible to have a cruise in one direction and jump on a scheduled bus to the other way. Besides, it is possible to travel only a fraction of the route: from Lahti to Vääksy Lock, from Lahti to Kalkkinen Lock or from Lahti to Kalkkinen Village. For example, you can book a cruise ticket from Lahti Harbor (9 AM) to the village of Kalkkinen (12 noon) and then back from Kalkkinen Village (3.50 PM) to Lahti Harbor (7 PM). Remember to check timetables.
If you travel to Lahti, Lahti Harbor is the must sight, at least in summer. Scenery is beautiful but there are also interesting sights and activities there. So I highly recommend Lahti Harbor.