The Japanese kitchen has a stunning convenience to some dishes, making them perfectly easy to eat while you’re in a rush or at work. Good examples for those kinds of dishes are Onigiri or colorful bentō boxes.Another convenient spectrum of dishes, even though they’re rather served in a fast food restaurant, is the family of Donburi (どんぶり) dishes, let’s have a look: 

Donburi is a Japanese “rice bowl dish” consisting of fish, meat, vegetables or other ingredients simmered together and served over rice. Donburi meals are served in oversized rice bowls, also called donburi. 
The simmering sauce varies according to season, ingredients, region, and taste. A typical sauce might consist of dashi flavored with soy sauce. Proportions vary, but there is normally three to four times as much dashi as soy sauce and mirin. For Oyakodon, dashi flavored with light soy sauce, dark soy sauce, and sugar is recommended. For Gyūdon, Tsuji recommends water flavored with dark soy sauce and mirin.

Donburi can be made from almost any ingredients, including leftovers.
The most famous Donburi there is, is definitely Gyūdon. The word Gyūdon, means literally beef bowl. It is a Japanese dish consisting of a bowl of rice topped with beef and onion simmered in a mildly sweet sauce flavored with dashi (fish and seaweed stock), soy sauce and mirin (sweet rice wine). It also often includes shirataki noodles, and is sometimes topped with a raw egg or a soft poached egg (onsen tamago).

Donburi meals are fairly cheap, delicious, fulfilling and can be bought nearly everywhere. The most popular chains for Donburi meals are Yoshinoya and Matsunoya. Keep your eyes open, those chains have a lot of restaurants all across Japan.