If you can travel to Hokkaido, the coastal and mountain areas have mild weather. Outdoor activities are not that tiring. My. Takao has a nature trail that is somewhat shaded. The yagyu nature trail is mostly under the shade of big trees. At nights, it's still hot and humid but you can have some fireworks if you find a place that allows it. I listed a few outdoor activities. Looking forward to hear from others about other stuff.
I recommend a few indoor activities, for those who don't want to stay out too long.