They are a one stop shop for almost everything. Although they are not all the same, 7-Eleven is amazing, as well as Family Mart. They often have ready to eat meals and snacks, as well as hot foods at the counter. They can be a useful place for sending mail, Amazon Japan often can deliver packages to them for pick-up, as well as access to photocopiers or ATMs if you need to take out local currency. It is seen as impolite to eat while walking or on public transit (except the Shinkansen, which is treated more in line to a flight).
100 yen shops are amazing (Daiso is a common and popular chain), Don Quijote is another great store for finding a variety of things from food, to electronics, to clothing and household and travel goods. You may want to pick up a small hand towel or washcloth as more often than not there is no means of drying your hands in public washrooms (electric hand blow dryers are starting to become more common though).
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