In Nagoya there is the Moku Moku Farm Restaurant on the 13th floor of the Nagoya Station Sky Building. It’s specialty is organic produce and there is no shortage of vegetables, fruit, meat, fish and desserts, all for just 1900yen at lunch time. You may have to wait, but it is worth it.
There is a chain of Korean BBQ restaurants called Chifaja around the city that has all you can eat meat and rice at lunch times for 1000 yen for 90 minutes (note: not all of them are open for lunch so check their website, and not available on weekends). Dinner is 2300yen
As of mid April 2015, there was a lunchtime all you can eat Vietnamese restaurant for 1000yen inside the lobby of the Kuretake Inn a few minutes walk northeast of Shizuoka station. Fantastic food.
Muten Kura Sushi, Sushiro and Kappa sushi restaurants
These three chains of sushi restaurants, whilst not all you can eat, but cheap enough to feel like one, have managed to streamlined the process of ordering sushi, and thus limiting operation costs and passing the savings onto the customers. You have a touch screen you order your sushi on, and it is delivered by either conveyor belt or machine. The money saved in wages means that each plate containing two pieces of neta sushi is only 100yen, meaning a meal of 20 plates will only cost you 2000yen (and trust me, 20 plates will make you full even after a long distance hike). These are in almost all major Japanese cities and large towns.