The wonders of Glimmerglass
Otsego Lake, nicknamed Glimmerglass by James Fennimore Cooper many years ago, is the source of the Susquehanna River and the reason Cooperstown was founded where it was. At nine miles long it is one of the larger lakes in the area and home to much of Cooperstown's non-baseball related activities. In the summer you can see numerous sailboats and motorboats out enjoying the waters. Activities that are easily participated in by visiting the local sailing club or renting a boat from nearby Sam Smith's Boat Yard.
Of course that's just one option. You can canoe or kayak if you're craving a bit more active activity, or you can simply go for a swim at one of several local beaches that are open to the public (check out Fairy Springs or Three Mile Point). The lake is also home to several species of fish that attract local fishermen. In winter the lake normally freezes over and you can see it dotted with numerous ice fishing huts as well as the occasional cross country skier.