I think aquariums tips, outdoor movies night, etc. sound like fun ideas. But one thing to keep in mind is how much of an "event" a hangout might be, and whether that might turn some people off.

It's like the difference between planning for a beach trip with friends versus just taking a spontaneous dip in the pool with your friends on a hot day: one feels more effortful than the other. And the beach trip, or planned events, might be the more fun choice. But after a long week, etc. a lot of people just want to do something social but more chill and effortless. Just being able to enjoy the moment with their friends. Which might be part of the reason why bars are really attractive for that kind of thing. You just go out, grab a drink, chill with your buds, and go home to relax. No real structure, more just down time.

Again, not dismissing those ideas. Definitely good in moderation. But just chatting with a friend over lunch/dinner, at a cafe. Hang out at the park at the end of the day. Things of that nature and effort should also be important. Even if it feels small, it can be the nicest part of friendships.