Tod & I and a large bunch of friends spent the day at a park bigger than a breadbox. We had to take the train out to Tachikawa to do it, but it was worth the 45 minute trip.
Showa Kinen Koen (Showa Memorial Park) used to be a military base. Now it's a giant playground. There's a series of shallow swimming pools and four waterslides; a mini-golf course, croquet lawn, frisbee golf course, boat rental, a bike trail, gardens & forests, a huge cargo net for climbing, and a series of bouncy trampoline hills.
If you want to splash in the wave pool or waterslide, hurry out to Tachikawa--the pool closes on September 4th. The rest of the park is open year round.
Showa Kinen Koen is so famous because of fireworks meet. Unfortunately, it has finished 30/Jul this year.