Any activity that is small enough that they need more people, but still large enough.

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)