I rented a kei truck for a trip in Kochi a couple of years ago - very cheap from the construction equipment rental service "k-center". No one-way rentals, unfortunately.
https://k-center.jp/

Forget karting around Tokyo, stinking the place up with two-stroke engines, while filming yourself in a stupid costume. Touring the inaka in a kei truck should go on the list of "real Japan" experiences. Misery and romance at the same time.