The first one has a water strider. I don't know why they use it.
The second one has a geometric design.
The third one has two imaginary creatures called "shachihoko" and they are symbolic items in Nagoya. THey are adorned on the roof of Nagoya castle.
The last one seems to show sightseeing places in Nagoya, though I have no idea which places are depicted.
This manhole cover can be seen in Assabu town, one-hour-and-half drive distance from Hakodate. The character comes from potatoes whose brand name is Mayqueen. Mayqueen potatoes are major product in Assabu.