so my weeb ass is finally going to Japan. Main things me and my partner want to do are Ghlibi museum and theme park, see the life size Gundam, and go to a baseball game. We'll be mostly in Tokyo and Kyoto and then the Ghibli theme park in Nagoya. We like all the nerd shit so we'll be in Shibuya, Shinkuk and Akihabara, and maybe do some fancy stuff in Ginza.
Akihabara is mostly focused on current release stuff. For older and second hand anime stuff you can go to Nakano Broadway. Ikebukuro is like Akihabara but aimed at women.
If you have any extremely niche interests, google for a store in Tokyo that caters to it. Almost certainly something for every specific hobby. I've seen a bespoke make-your-own ink shop for fountain pen enthusiasts.