Convenience stores are a good measure of the general cross-cut interests of a society/people. In Taiwan they smell of medicinal tea eggs. In the United states it's hot dogs and nachos. India doesn't really have them yet, but convenience (konbeni) stores have been fully integrated into Japanese society to a degree not seen anywhere else in the world.
It is common for young people, especially students and single males, to say they live a 'convenience-store lifestyle' (konbeni seikatsu), meaning they basically get most of their nourishment from the 7-11s of Japan. Lawon Station, with the nostalgic milk jug logo (blue and white) is the largest chain, though Family Mart (sun and moon logo) and Circle-K are also common.
Convenience stores are primarily good for two things--easy food/beverages and magazine/Manga browsing. For quick snacks the 'Onigiri' are often the best bet. These are 'triangles' of rice compacted around salmon or picked plum, chicken, konbu, etc and wrapped in nori (seaweed). Once purchased (¥100) don't just rip the wrapper like you might do in America, but carefully note how the nori is kept separate (thereby crisp and dry) from the rice, and how if unwrapped properly (meaning following steps one, two, and three) the nori is suddenly found magicaly surrounding the rice. It's amazing, but can take a try or two to get it right.
The beverage choices are vast, including numerous kinds of hot and cold tea or coffee, a Gatorade-like sports drink called Pocari Sweat, and a fizzy sweet yogurt drink Calpis soda. The magazine/manga rack can give you intersting insight into popular culture reading interests, and feature Manga for all strata of Japanese society. The mens and ladies fashion magazines are also worth checking out, and (as is true in bookstores) the management doesn't mind.