In Japan, and Japanese influenced-countries like Taiwan, you can get sandwiches at the ubiquitous Lawson Stations or 7-11's, BUT you are missing out on a treat if you just stick with the Western food. Try an onigiri for a light, healthy and very tasty treat. About the size of a baseball, the standard triangular onigiri you can get in convenience stores is also an incredibly well-designed feat of packaging.
The central rice "ball" has a small amount of filling: in this case, if I read my Chinese right, it's creamy shrimp(?) - please correct me if I'm wrong here. But the crispy nori wrapping goes soggy if it touches the rice. Several years ago, Japanese design engineers came up with a way to keep the nori on the outside separate from the rice by a two-layer packaging design. You pull the tab (top dead center of the triangle) out and around, splitting the package in two, then pull off the plastic wrapping from each of the other corners, and voila - crunchy nori wrapped around the riceball. It's like the magician's trick where he pulls the tablecloth off the table and leaves the crockery intact.
Try two different flavors and a drink and you've got a great lunch for around USD$5.
Safe travels! Des