Toshima-en is supposed to be quite good:
http://gojapan.about.com/od/themepar...s-In-Japan.htm

There used to be a place with some awesomely scary water slides in JoeJones' end of town ... but I heard it closed down. Strange, since it was extremely crowded when I was there, but I guess these places are only super-crowded for a short season.

If you want somewhere to escape the heat, you could also just take a train to the mountains... a day of walking and a hot spring dip is a nice way to enjoy the summer.