I have 5 days/4 nights between Kyoto and Tokyo. Looking for recommendations for nature/adventure

Hey brainstrust,

we are a family of 4 with two boys then aged 10 and 12 travelling to Japan for two weeks late September/early October.

We have 4 nights in Kyoto (flying into Osaka) booked and 5 nights in Tokyo (flying home from there).

In-between, we have a gap of 5 days and 4 nights and would like to get out into the countryside, go for little walks/hikes, we love kayaking, we'd love to see rural Japan and spend time in nature.

One thing we kind of have our hearts set on is the Nakasendo between Magome and Tsumago (I realise this is a busy section).

We're happy to rent a car (we're from Australia so no worries driving on the left, I know about the IDP).

Please hit me with your suggestions or ideas?
Kayaking, walks, homestays, over night kayaking trips (if this exists..)

Thanks!