There is very little in the way of sights in Korsakov, although there is a great view over the harbor, along with an uninspiring monument, on the hill just behind the sea terminal. The main drag, Sovetskaya Street, is lined with small street stalls during spring and summer months, selling fresh fruits and vegetables. According to a wikipedian, the strawberries are particularly good.
Korsakov History Museum
A small museum with exhibitions about the Japanese possession 1905-1940.
Old Hokkaido Takushoku Bank
This crumbling building at the southeastern corner of the city park is one of the only visible remains of the Japanese possession of the city. Efforts to convert the building into a museum have yet to bear fruit, and the doors and windows have been barred.
Outside Korsakov there are a few beaches at the hamlets of Okhotsk about 1 hour and Prigorodnoye about 30 minutes which also host a few Japanese war monuments. Access to both of these sights requires a car or taxi.