I think there are lots of options.
1. Board game venues. They're becoming more popular, and yes, they exist in the US. I would be surprised if there wasn't one in any major US city.
2. Escape rooms. I think there are at least 50 in my city. They're literally everywhere if I search on Google Maps.
3. Cafes. If you're looking for something low key, you should be able to find some cafes with some hot wine, snacks, and couches for relaxing with friends.
4. Pubs. You can search for one with pool or darts if you feel like doing an activity while having casual drinks.
5. Meetups/Couchsurfing events. If you want to get together with a few dozen random people, there are usually events on both of these sites.
6. Rooftop or outdoor movie venues. They're a nice change from the usual movie theater if you want something a bit different.
7. VR experiences. These are starting to pop up as well. They have a bunch of rooms with headsets, and TVs showing what the person with the headset is seeing. So, you rent a room with friends, one person is playing, others are watching on the TV. Or, there are VR experiences where everyone puts on a headset, and you run around a VR warehouse playing together.
8. Lots of random activities that have existed for ages, like bowling, ice skating, go-karting, etc.
9. If you want very low key just go to a big book store, get a coffee, and read some books.
In short, I don't think the "young adult nightlife experience" is stagnant, and I think it's probably more interesting than ever before, with lots of alternatives to clubs or loud pubs.