Fiona here, with a few suggestions for outdoor activities in Kumamoto.
Go Nature offers kayaking, SUP (Stand-Up Paddle boarding) and river trekking in various locations around the prefecture. Their new splash trekking tour also includes an optional onsen visit. While I haven’t tried that yet, I can vouch for the Ezu-ko sunset and Midorikawa Dam kayaking tours as being very fun with friendly and professional guides. https://go-nature.club/
Forest Adventure Misato has zip-lining, Tarzan swing and ropes courses in a beautiful setting with camping also available nearby. This place has great activities on offer for both children and adults. https://foret-aventure.jp/park/fa-misato/
It’s also possible to go zip-lining by the sea in Amakusa. I haven’t tried this one out yet but I’m sure the views are amazing. https://tenku-zipline.jp/
Thrill-seekers have the opportunity to try paragliding, bouldering, climbing, cycling , trekking and buggy rides in Aso with this company. https://asonatureland.com/activity/
If anyone knows of other fun outdoor activities for the summer months, please post them in the comments.