Demachi, Japan + Add a trip

KitoKito Sushi. It's a local kaiten (conveyor belt) sushi place, with plates starting at 100yen. You sit at the counter, take what you want, and pay at the end. You get a good amount of fresh sushi for 500yen, or you could get better quality/more expensive fish and go as high as 1500yen before getting full.

Another similar place is Megumi, but I've not been there in a long time, and can't reliably comment on it.

If you're just on the go and want something cheap, convenience stores are you friend.