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I'm so jealous ;)

Conveyor belt sushi - UOBEI: https://www.genkisushi.co.jp/en/search/prefect_list.php?category_id=3&prefect_id=&city_id=

But even in stores like 7eleven or lawson, department stores or supermarkets, you can buy really good sushi and a lot of other food.

Ramen: https://www.tripadvisor.com/Restaurant_Review-g1066456-d1680997-Reviews-Kyushujangara_Harajukuten-Shibuya_Tokyo_Tokyo_Prefecture_Kanto.html or Ichiramen

All you can eat sukiyaki or shabu shabu in Nabezo: http://nabe-zo.com/lan_en/restaurant_search_en/

And there is a lot of great street food in Asakusa, Harajuku, Shimokitazawa, everywhere ;)

For Osaka I don't have recommendations, but if you go to Kyoto (and you should, because it's beautiful there) I recommend Nishiki Market and for Ramen: https://www.tripadvisor.com/Restaurant_Review-g298564-d7122319-Reviews-Musoshin_Gion-Kyoto_Kyoto_Prefecture_Kinki.html

Takoyaki in Gindaco, this is a chain and you can find it everywhere in Tokyo.

There is a lot of delicious food in Japan :) Have a great time!

Wow, thanks for all these recommendations, that will certainly be helpful! I'm really excited for Japan :)