Be careful, males and females are in general attracted to different activities. (I don't know why). If you are a guy trying to meet girls (or vice versa) your first choice is probably bad. If you don't see someone who could be a match right away you are in the wrong activity. (this is before you meet them to learn if they are married or have have compatible personality)

Certain sports qualify. Volunteer groups can qualify. Some hobbies work.

For sports today the rock climbing community is very inclusive and the sport naturally causes people to get to know each other since you can't climb with ropes in most places without pairing up with someone. The whole gym culture of rock climbing didn't really exist yet when I was in my early 20s.