If you like sushi, try it at Tsukiji market.
Curry shops around Jimbocho station.
In the summer, I recommend you try zaru-udon or zaru-soba, kind of cold noodles, better to eat with Tempuras. I usually eat at Marukame Udon restaurants.
The others are Gyudon, Oyakodon, Motsuni, Miso Ramen or Soy Ramen (if you like more meat you should choose Chasiu Ramen),...
Hope you will enjoy your stay in Japan :)
You can consider going to a local supermarket and pick up some fresh fruits, for example, peaches and grapes. They may cost more than you are used to pay for the same types of fruits in your own country but you will not be disappointed (I hope) with the quality.
I would definitely recommend "Tendon" which is deep fried seafood/vegetable on a bowl of rice.
If you're in Tokyo, I definitely reccomend "天すけ" (Tensuke) in Koenji where you get to see the Tempura fried and served right infront of you. Go for the Tamago Lunch, you won't regret it :).
However be sure to get there by 11:15 because when I was there a long line has already formed even before the shop is even open!
I would like to recommend to eat Takenoko no sato.
It is one of famous snacks in Japan, it is made of chocolate and cookies. It cost about 200yen.
If you eat it, you will get sweet and lovely taste.
It is amazing.
There is also different version, it calls, Kinokonoyama. It's also great.
Sushi is the must of course but if you are more into like local food, try to go to Izakaya which is like a japanese version of tavern. You can experience so many kinds food that most japanese people eat usually with very reasonable price!