The most common motif by far used in the designs on manhole covers in japan is the cherry blossom. Considering the central place occupied by rice in Japanese identity it is surprising that it does not appear more often than it does. This first one is from Mizuho up in the mountains near where Iwami meet Hiroshima.
I found this second one in the village of Koshita south of Usa in northern Kyushu.
Also in northern Kyushu, but on the opposite side in Fukuoka, this one is from Itoshima, one of the very ancient centers of early Japanese intereactions with Asia.
The final one is from near Kurayoshi in Tottori and it shows a farm woman using a senbakoki, a threshing machine with a steel "comb" that separates the the easr and grains from the stalks. Prior to its invention in the 17th Century a tool made from a piece of split bamboo, a kokibashi, was used.