My recommendations for Japanese trail food are as follows:
Oreos – Enough said.
Meiji Dark Chocolate – The cheapest brand, and the best tasting in my opinion
Ramen Noodles (various brands) – These are light, packed with calories, and ready in minutes.
Pre Boiled Eggs – You can carry these without fear of them breaking, they are a great source of protein and compliment ramen extremely well.
Soba Noodles –They are made from buckwheat and contain around 13grams of protein per 100grams. They also only take around 5 minutes to cook, making them both nutritious and convenient.
Japanese Curry – These packs of curry may be a little heavy as they contain water, but they do add enough flavour to any meal to make up for their weight. You put the packs in boiling water for five minutes then empty them onto either rice or noodles and you have a delicious meal.
Beef Jerky – They sell this in most convenience stores, and although it is not the cheapest it does hit the spot when you’re in need of protein on the trail.
Dried Squid/Octopus – For someone a bit more adventurous with food, and why not, you’re in Japan after all! Great source of protein you don’t have to worry about going off, and it’s cheaper than beef jerky.
Vegetable Juices – These are in every food store and go for around 100 yen a drink. I would try to have one of these per day to mitigate the effects of not eating fresh fruit and vegetables