1) Manju (Nikkuman/Pizzaman): Throughout the year (summer excluded), convenience stores will stock piping hot Manju (steamed buns). Typically, they'll have a variety available, but unless you're an adventurous eater, you should stick with either the Nikkuman (pork bun) or Pizzaman (marinara and cheese bun). If you want to make sure some are available, the Nikkuman steamer is typically found on the counter. Nikkuman is always white, and the pizzaman is typically yellow. The average price will range between 100 and 200 yen. Either one of those make for delicious choices to fight the hunger. (WARNING: the Circle K Sunkus Pizzaman have corn hidden inside. That's a dealbreaker in our household!)
2) American Dog: If this looks suspiciously like a corn dog, that is because it basically is. The only difference is that the breading is typically more crispy and less dense than its American counterpart, and the meat tastes more like a sausage than the hot dog you typically find in the states. When you order one, the staff will typically give you a small ketchup and mustard dispenser that is quite ingenious. Simply squeeze the two ends together and it will drizzle out slowly--no need to worry about a big mess. These typically cost about 150 yen.
3) Croquette: Japanese Convenience Stores now typically offer a wide range of fried foods. The best option for staving off hunger while spending less is the croquette--a fried potato patty usually mixed with vegetables and/or meat. Most convenience stores will serve this to you in a little to-go bag that conveniently has tear-along seams to allow you to eat it from your hand without making a mess, and they only cost about 60-100 yen.
4) Onigiri: These traditional Japanese rice balls are conveniently packed and come in a wide variety. It can be daunting to try to guess which ones would be best to eat, but your safest bets are tuna salad or salmon (fortunately most packaging has images of what is inside the rice). These will run you between 100 and 200 yen.
5) Sandwiches: Convenience stores will almost always have two kinds of sandwiches stocked: (1) egg salad and (2) Tuna salad. If you are anti-mayonnaise, you will be out of luck, but if you are just looking for a quick bite, these 150-200 yen sandwich packs will give you the extra boost you need to keep your energy in your travels. Simply look in the refrigerated section of the convenience store.