Bars are notable for staying open late, but I’m not really familiar with alternatives that might operate similarly. I think many cafés (at least around my area) tend to close a few hours earlier by comparison, which would be my go-to for an alternative otherwise, so what other options are there?
It really depends where you are - some places have a lively late night culture, others not so much.
All night diners, 24 hour fast food places are really the only alternatives here (though you could count movie theatres with late showings, I guess).
But (unless you have personal reasons for not being around alcohol and drinkers), you can totally go to a bar with a vibe you like and just hang out drinking soda. Bonus points if that bar has something cool available aside from just booze (barcades, live music venues, etc.).
If you’re not looking to specifically be around people, you could also just check out some green space outdoors, but keep your wits about you and use your judgement - some places are lovely to chill at 3:00 am, some would be if it wasn’t for the sketchy folks shooting up.