Why does it have to not cost anything? If you can spend $5 you open up entire categories of businesses that exist specifically as adult hangout spots: cafes, bars, wine bars, board game cafes, whatever. You said you're doing FIRE but also are a SWE. Surely you can pick up a side project or something and pay for like an entire year of these hangouts for a few days of work. Your mental health is worth it, no?

$5 for a few hours of hanging out in a safe and curated space really isn't a bad deal at all (it might be for the business, but not for you).

Back when I was in a similar boat, spending time at these community spaces and just being among people, whether I was working or reading or goofing off or people watching, really helped keep me sane.

If you can spend a little more, $10 or $20, it also opens up bowling, golf ranges, climbing, various lessons, concerts, lectures, etc.

The hard part here isn't the solo adult activity, it's being too cheap to support businesses that provide those activities.