Oswego sits where the Oswego River meets Lake Ontario, and the water explains almost everything about it. The city was fought over, fortified, traded through, and lit by lighthouses because this harbour mattered on the inland route between the Atlantic world and the Great Lakes.
The best day here is compact but strong. Start with Fort Ontario, then add the Safe Haven Museum next door for one of the most unusual World War II stories in the United States. Down at the harbour, the H. Lee White Maritime Museum and the west pierhead lighthouse pull the city back toward the lake.
Oswego is not polished Finger Lakes prettiness. It is a working lake port with good history, wind off the water, and enough specific sights to reward a deliberate stop.