When I'm looking for an inexpensive but filling snack, I typically go for an onigiri. It's a rice ball that's often wrapped in seaweed and is stuffed with ingredients like salmon, tuna mayonnaise, and umeboshi (salted Japanese plum).
They typically cost about $1, so they're easy on the wallet. If I'm staying at a hotel where breakfast isn't included, I'll often get one or two the night before so that I have something to munch on the following morning.
Most onigiri sold at Lawson are wrapped so that the seaweed is kept separate from the rice, so even if I buy it the night before, the seaweed is still crispy the next day.
However, the rice-to-filling ratio with these onigiri admittedly lean far more to the former. When I want a little less rice, I'll opt for a temaki (hand roll) instead.