"Family restaurants" are an economical way to eat in Japan. These are comfortable, spacious, and offer plenty of amenities. One of these is always an open bar without alcohol (if you want to ask in Japanese, it's called "dorinku ba") that offers unlimited refills on all kinds of drinks: soft drinks, tea, coffee, etc for a very low price (usually less than 300 yen). Many people go to these restaurants to study, bringing their laptops. They usually have large tables, and are a good place to bring children, as they always have a special kids' menu and games.
You can find different varieties - Italian, Chinese, international, etc.