I prefer to know what I will do when I'm "hanging out" with someone, even if it is "hey, lets get some coffee and talk."
I think there is a couple of reasons for this. Unless I am *really* comfortable with someone, I will have some anxiety about getting together with them. Not sure about their motivations, not sure what to expect, etc. So having an idea of what is going on ahead of time reduces some of that.
Another thing is that I've had so many experiences of ideas of "hanging out" falling through that I don't really believe someone who says, "let's hang out." If you say "let's go do such-and-such specific thing" then I am more likely to take you seriously that you actually do want to spend time with me. When there is some activity like a comic convention on the calendar, you are less likely to blow me off.