Food, Drinks, Utilities- Restrooms - convenience stores is a small world in itself..
Convenience stores will store the following -
1. Food - salad boxes, ready to eat bento box (pasta/curry etc) (mostly non-vegetarian), fruits (banana, apple, oranges), Curd, Diet biscuits, nuts, chips, chocolates, chewing gum, cup ramen noodles, soups, french fries or chicken nuggets (ready to eat), plain rice
2. Drinks - Cold drinks, Japanese tea, Diet supplements (proteins, vitamins), hot coffee, alcohol (only some stores with a drink mark), milk, butter milk, energy shots (so you get refreshed or to overcome vomit feeling after over-drinking etc)
3. Daily used things - shirts, necktie, kids diapers, sanitary napkins, shaving cream, razor, stationery (paper, gum, eraser, pencil, pen, diary) etc
4. Magazines & Comics - a huge selection of magazine. English newspaper - either Japan Times or Yomiuri (some keep Wall street Journal)
5. Utilities - Bank ATM, Concert ticket vending machine, photocopy machine, fax machine etc
You can also step in to ask directions and if not crowded, the staff will kindly guide you as well (especially if you are driving out on countryside and get lost - you can buy a coffee or something at such shop and then ask for directions as well)
Some stores will also have a restroom /toilet - so if you are with elders or kids then this is of help. To show your thankfulness, you can buy again some small thing there.
(for vegetarians - you can take plain rice, curd, butter milk, plain milk, some chips (check the backside of the chip packet as it will show ingredients - sometimes chicken powder, sea weed powder is added and hence might not be recommended for you) or take diet biscuits like Calorie mate - so that you dont starve and store yourself with necessary vitamins - proteins)