Shinjuku is where Tokyo reveals itself as a city that operates at a scale and intensity that has no direct comparison. The east exit of Shinjuku Station, the busiest station in the world, opens onto an intersection of department stores, electronics shops, and pachinko parlours. The west exit opens onto the skyscraper district and the Metropolitan Government Building, whose free observation decks give the fullest aerial view of the city. Both exits deposit you into something overwhelming, and the correct response is to pick a direction and walk.
Kabukicho, Tokyo's entertainment district, occupies the northeast of Shinjuku and is dense with hostess bars, karaoke rooms, game centres, and everything else the nighttime economy requires. It is functional rather than dangerous, and interesting in the way that large organised pleasure districts are interesting. Walk through it once.
Golden Gai is the thing that justifies Shinjuku as a destination in itself rather than just a transit point. It is a grid of six alleyways from the 1940s, miraculously undemolished, containing approximately two hundred bars, each one seating between six and fourteen people, each with its own theme, clientele, and house rules. The bars that have foreign visitors are marked. The bars that do not are also marked. Go to a mix of both.
Every first-time visitor to Tokyo for at least one night, because Shinjuku is the city concentrated. People who want to drink in an interesting way: Golden Gai rewards curiosity and rewards it differently from anything else in the world. Anyone who wants Tokyo's full urban intensity, including the parts that are overwhelming.
Not the right base for anyone who came for the historic or neighbourhood Tokyo. Shinjuku is fast and loud and designed for transit and consumption. Yanaka and Shimokitazawa are an hour away and a different planet.
Golden Gai: enter from the Hanazono Shrine side. Pick a bar with a maximum capacity sign you can deal with, sit at the counter, and talk to whoever is next to you. The Metropolitan Government Building observation decks on the 45th floor, free, open until 11pm on most nights, the best skyline view in the city. Omoide Yokocho, Memory Lane, the smoky skewer lane behind the station west exit, which has been there since the 1940s and still operates as if nothing has changed. Isetan department store on the east side for the basement food hall, which is a proper tour in itself. Shinjuku Gyoen if the cherry blossoms or autumn leaves are in season: the biggest formal garden in Tokyo, and completely different from everything around it.