Seibuen Yuenchi Pool

This large and popular theme park lies just north of the Saitama-Tokyo border, and is only 50 minutes from Shinjuku by train. The park itself is known for its quality roller coasters and its outdoor ice skating rink (winter only, natch). The outdoor pool, open during the summer, covers a large area and features no less than four tubular water slides, the obligatory wave machine and plenty of shallow splashing spots for the littleuns. It's not cheap, but neither is it a bad price for a day out in the Tokyo vicinity when all's said and done – and for an extra ¥500 you can get admission and free rides at the adjacent amusement park.