just have a look at ojiya. there is the koi-museum, who was mentioned by joe.
also you find koi-mosaik on the streets, drain caps with koi-pictures and the station has stairways which look like a big koi.
streets in yamakoshi are very dangerous in wintertime. there is a lot of snow, so there are high walls of snow on each side of the single track. suddenly there are other cars or a big snowplow in your way and anyone doesn´t know where to go

... i´m always happy not to sit behind the steering wheel! but while you will go in april, i think it will be confortable....
have a nice time!