The top tip is: just go. It won’t be the futuristic Babylon that you expected but more the projection of the 80s which dreamt of it, and it will be unparalleled (also might not stay this way forever).
Walking down some well-documented neighborhoods -Shibuya, Akihabara, Ikebukuro, Shinjuku and the like- is enough to dose you on vertical neon, food stalls, high fashion, sararimen, shrines, crazy trinkets, nylon umbrellas. Assuming you’ll evening stroll through one or two of those, here are a few extra, focused, things to look up or under for.
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Alleys jammed with tiny izakayas, real sararimen nests.
Celebrated ones: Piss alley in Shinjuku, Nonbei in Shibuya, around and under the railway bridge at Shimbashi.
The artificial islands and the bay
Where you realize that everything solid in your field of vision is manmade.
Radio Center electronics mall
Radio Center is a sensation. A small passage close to Akihabara metro station (35.698316, 139.771861). And an old, half-used, three-storied narrow building next to it. Grab it while you still can.