Sendagaya's Metropolitan Gymnasium has two pools: a 50m, eight-lane Olympic one and a 25-metre, six-lane pool. You can use them plus the weight training room for ¥700. Fitness classes cost extra, with prices starting from ¥550.
Open: 9am-11pm, Sat until 10pm, Sun & hols until 9pm
Admission: 2.5-hr gym pass ¥700, junior high school students and younger children ¥300
Make sure to bring a swimming cap.