Question: what can i buy with 1 yen?
Answer: Small pieces of nuts, bolts, washers and gears, etc, maybe many of those in Akihabara. 1-yen Pachinko (pinball like machine for gambling). They are around any big stations. You can get one ball to throw in the machine for 1 yen, although I think you have to spend 1000 yen minimum to get 1000 balls.