It's really hot in Summer in Tokyo and I went to a retreat to the Nikko highland. There is Not so close to Tokyo ant it takes 4 hours from train and local bus.
There is 1,400 meters elevation and very cooler than Tokyo.
I had camping 2 nights and also fishing in "Yunoko lake" and "Yukawa river". Several years ago, I got some trout in both lake and river but I caught no trout in this time... :(
There is not only really beautiful nature but also very close to historical monuments that are world's heritage.
I recommend traveling to Nikko if you wish to go to see monument and nature.