Udon
Udon is one of the noodles often eaten in Japan, like Soba noodles or Ramen noodles. Udon is more of a thicker and bigger style of noodles, they are used as casually as any other type of noodle in Japan
You don't need to wait more than 3 mins to have it on the table. Some restaurants are specialized in udon. Hanamaru udon is a chain restaurant that sell udon, there are these stores all around Japan and some even in other countries.
Soba
Soba noodle in Japan is really unique. You have to "swallow with sound" and it's a polite way, "good manners" for the noodle meals - you may be astonished. Each Soba place has its own "Dashi" soup and it's used for every plate so that any dish is coordinated into a similar tone as one. It is hard to mention one shop or chain because Tokyo and Japan are full of Soba restaurants.
Takoyaki, Octopus Balls
Takoyaki is originally a food from Osaka, but it has become well known all around in Japan and in Tokyo there are many places that sell takoyaki. Takoyaki itself is pieces of octopus backed and friend in to some batter and when ready, the balls that are made usually mayonnaise and sauce is put on top of them and then they are ready to eat. Though be sure to be careful they are very hot when ready.
Mos Burger
Mos Burger is a Japanese hamburger restaurant chain, with many of the burgers bit more Japanese style, with what they include in them as well as the sauces used. For example, shrimp burger or a Teriyaki burger, though in most of the hamburger chains in Japan do serve some sort of a teriyaki burger. Mos buger serves very cheap though bit smaller burgers. The cheapest burgers starting from 220 yen
Beef Bowl (Gyūdon)
Beef bowl is a dish where strips of meat is served on top of some rice. You can have different additions to the bowl for extra price, for example some of the most common additions is a raw egg. There are quite a few beef bowl chains in Japan, like burger chains in US. They are competing in price cut each other, and the news of it is already a regular seasonal tradition which economic paper features a lot. Yoshinoya is one of the oldest and most famous and Beef Bowl chains in Japan. The cheapest bowl starts at 300 yen, so it is very cheap compared to even most fast food places.
Curry rice
The Japanese curry rice differs from the Japanese one a lot. The basic version of Japanese curry rice is the Japanese curry, potatoes, onions, carrots, meat and this curry sauce is served with rice. Another version is the curry sauce and rice served with a Tonkatsu (fried pork cutlet). This is another of the many fast foods in Japan and there are many different restaurants and chains selling curry rice in Tokyo. One of the most well kown is Curry House CoCo Ichibanya that has restaurants all over Japan as well as other countries. The prices of the meals starting around 600 yen.
Tempura
Tempura is another famous fast food in Japan. Tempura is usually fried seafood or vegetables. The most well-known and usual Tempura is a bowl with rice and on top of it some batter fried prawn.
Ramen
Ramen is of course one of the most well known fast foods in Japan. Though ramen comes originally from China, Japan has it made it in to its own dish with lots of different versions made of it. There are Ramen restaurants all over Japan and Tokyo as well. The costs of the ramen restaurants differ a lot as well, but it is nice to try many different restaurants and many of the different restaurants.
Okonomiyaki
This is another dish that originates from Kansai region (region that Osaka is part of) but it is available and famous all around Japan. Okonomiyaki is kind of mix of a pancake and an omelet it is egg and batter mixed together and in to the mix the other ingredients are mixed in to, then it is poured and cooked on metal pan from both sides. At many of the Okonomiyaki restaurants you choose the mix or all the ingredients you want in to the batter and then you cook the okonomiyaki yourself. Usually on top of Okonomiyaki there is poured mayonnaise, Japanese style Worchester sauce, seaweed flakes and at times pickled ginger.
Monjayaki
This dish is very much like Okonomiyaki, but it is mixed in to a liquid ingredients and cooked by yourself like Okonomiyaki it is cooked usually by yourself on a metal pan that is usually on the table. But it is not as much of an omelet, because you eat it out of the pan on the table. You eat it piece by piece. This dish is also said to look like puke and called as such because of the appearance before it is cooked, but you should not worry about this, it is truly a delicious dish.