Some places I go to in Japantown:
For ramen, Marafuku has very good tonkotsu ramen, there is usually a waiting list. There is a branch of Hinodeya near the Japan Center as well, they specialize in a lighter dashi broth and have outdoor seating.
Nari [2] is near the Japan center in the Hotel Kabuki. Innovative Thai from Pim Techamuanvivit. Good cocktails too. Their lounge/bar area serves the full menu and is for walk-ins.
For quick conveyor belt sushi in the Japan Center I like Tenroku in the Kinokuniya building. Not high end at all but scratches the sushi itch. Like most conveyor belt places it’s probably better to go when it is busy. On the other side of the mall is Izumi which I believe is run by the same ownership. Where Isobune used to be. I believe Izumi has outdoor seating.
For higher end sushi there is An which I liked a lot, but I haven’t been since before the pandemic.
And a little farther away if you walk down Fillmore are the State Bird affiliated restaurants - State Bird Provisions, The Progress, and The Anchovy Bar.
Also for sweets, there is a branch of Mochill Mochi Donut in the mall which is worth a try.
Other restaurants near the Japan Center that I want to try but haven’t yet:
Daeho Kalbijjim & Beef Soup - I’ve gotten good delivery from their location in the SF H-Mart, their seolleongtang was really good.
Yuji - opened recently, Kappo style restaurant.