Tokyo Summerland - Akiruno-shi
The first thing we did these holidays was head to Tokyo Summerland Water park. It's about 30 minutes from Kokubunji, and is a HUGE place with indoor and outdoor areas. Highlights include speed slide, obstacle course with ropes and a ~current~ pool that you can float around in endlessly.

Although there was a lot of people, it was still possible relax and enjoy one's self. So much to LOOK at too ;). We stayed for about 4/5 hours and then played 'golf' at the driving range next door.