Elaborate onigiri is king
Traditional onigiris have a nice healthy appeal, and retain the "classic" Japanese dietary elements, such as rice, fish, and pickled veggies, but let's not limit ourselves to just the classics. Every convenience store has its own brand (or at least brand that is made for that convenience store), and sometimes the creations are quite amazing and delectable. Some of my personal favorites make use of meat patties and mayonaise.
Basically, anything that looks like it has a patty on top of the rice is going to be a heavy hitter. The onigiri itself will be in a short cylinder shape, and a flat patty of either sausage, hamburger, fried chicken, or fried potato may be on top of it, with a dollop of delicious mayonaise in the center. And of course, a strip of seaweed around the outside edge to secure the concoction in place. It is Japanese junk food that wouldn't have been thought possible in classical times, but if you want a quick snack that will fill you up, this is the way to do it.