While I love WW2 history, and lots of cool places in the south around Hiroshima and Nagasaki... southern Japan is a hot, sticky, non-air-conditioned-mess in the summer. Even Tokyo can be oppressive, especially in some summer months due to the lack of power grid capacity that still has not been repaired since the tsunami and the nuclear plant meltdown.

My recommendations:

- 3 full days in Tokyo, with the willingness to stay up late to hit the various night life areas with restaurants and crowds (Shinjuku, Fish Markets, etc. - decent list of night markets and flea markets here https://wanderlog.com/list/geoCategory/869390/best-night-markets-in-tokyo)
- 2 days - Make your way north and do some of the hikes and hot springs https://www.alltrails.com/japan/hokkaido/hot-springs
- 1 Day Sapporo
- 1 Day Sapporo to Kyoto
- 1 Day Kyoto, evening train to Osaka
- 1 Day Osaka