Lake Sayama, also know as the Yamaguchi Reservoir, is a man-made lake used as a water supply for Tokyo. Lake Tama is also close by.
As it's surrounded by trees, there's alot of wildlife in this area. There's also a cycling trail all along the lake, which I've never used, but am very interested in. It should be really nice in the spring, seeing the cherry blossoms along the path and feeling the cool breeze.