The cherry blossoms start blooming, and much of the countryside is covered in pink flowers. This season extends into June, depending on the area. Summer, as I explained, is very hot and humid, but not totally unbearable. Riding in the city with a full-faired bike is pure hell, as I found out one very hot day in August in Osaka a few years ago. The third week in August is the time of the Obon festival, and things will be crowded, as almost everyone in Japan is on holiday.
Days are pleasantly warm, there's no rain, and the leaves are changing color.
Beaches will be uncrowded, and tourist season is winding down. Route 338 in Tohoku. Brand-new pavement, totally deserted. I arrived in Japan in 1989, I used to leave my helmet on my motorcycle handlebars all day, unlocked, and more importantly, unstolen when I got back. Back in 1989, I thought I owned the only u-lock in Japan.