You can enjoy Tokyo nightlife alone. Start by deciding what you want to happen once you get inside. Meeting other people, listening to a band and spending an hour on a rhythm game call for different places.
Being alone is rarely the most useful search filter. “A gig I can enter tonight” or “a hosted event where people expect introductions” gets you closer to an evening than a list of entire nightlife districts.
If you want someone to talk to
Choose an event that actually organizes introductions. For example, Tokyo Pub Crawl's own FAQ welcomes solo arrivals and describes hosts and group activities. It currently lists Friday and Saturday evenings starting at 7 PM in Roppongi, with exact arrangements on each event page. It is a 20-plus nightlife event; check the dated listing before paying.
This is a concrete option if you want a group with a shared plan. It is also a party-oriented product, so read the itinerary before deciding it suits you. Booking a crawl because you want a quiet conversation about Japanese cinema would be a peculiar way to solve that problem.
An ordinary bar gives you more freedom over the pace. Ask for one seat, order, and let any conversation develop. The useful distinction is what you are buying: a drink and a place to sit, or a hosted social event. A bartender being friendly does not make introductions part of the ticket.
If you want music and would rather skip introductions
A live show gives you something to attend from the moment you arrive. Open a venue's schedule, listen to the listed artists and choose a night for the music.
The Flowers Loft calendar illustrates why the actual event matters. It lists daytime, evening and late-night performances, each with its own opening time, ticket channel and price. The venue name alone cannot tell you which kind of night you are walking into. Our first livehouse concert guide explains the extra drink charge and numbered entry.
For a seated show, Blue Note Tokyo's reservation guide lets you select the performance and seat, with food and drink accounted for separately from the online seat payment. Look for availability for one person on your date. A seat shown for a pair is not automatically a solo reservation at half the price.
You can have a very good night without getting anybody's contact details. Choose a performance you would enjoy even if nobody spoke to you.
If you mainly want to get out of the hotel
An arcade is a low-commitment option: choose a game, pay for a play and decide whether you want another. Taito's visitor guide covers cash and supported cashless machines. Check the particular store's game selection rather than assuming a building full of crane games also has the rhythm game you came for.
Akihabara after 8 PM is a useful starting point for this sort of evening. Shops and arcades keep different hours, so shopping can turn into playing as the night progresses.
Before leaving the hotel, save your first stop and the route back. If your plan depends on trains, check the last workable connection rather than guessing from the time you expect to leave. If you miss it, the next decision is transport or somewhere to spend the night. You do not need to commit to dawn just because you went out by yourself.