You see these places by the river all over Japan. Little hole in the wall open restaurants.
There is one in Otawara: Map Location
The first time we went here was in mid summer. Here's how it works. You go up to a window and there is a Japanese menu on the wall. For us with our basic Kanji reading and good Kana it was the most impenetrable menu ever. But we finally managed to place an order. You have to know what area you are sitting in and tell the lady your table number when ordering. (hint, local river fish are called Aiyu. You want to eat them!)
There are some guys with a charcoal pit cooking fish on sticks and thats what we ordered. It was very good. We understood from them that we should come back in September when they will be catching fish in the river and cooking them right there.
This we did, and on that trip they had set up a giant fish trap on the river. The fish was fantastic. It cost 2000-3000 yen a head for a hole in the wall restaurant. But its worth it.
Its also possible to get your own charcoal pit with seating area around it. For a group that would be fantastic.