One of my favorite restaurants in Japan is “Coco Ichiban” (not to be confused with the Coco’s family restaurant chain). It’s basically a Japanese curry restaurant. I mostly like it because the curry is good, you can get many different toppings, it’s fast, and it’s cheap.
Also it’s one of the few restaurants in Japan that is 100% no smoking (this is very rare indeed). They have menus in many languages, so even non-Japanese can comfortably order on their own. You can also choose your own level of spiciness, ranging from the mild level 1 to the fiery level 10. My goal is to try the level 10 someday. Currently I usually enjoy a level 5 (which is quite spicy in it’s own right). I highly recommend trying it sometime.