I usually respond well to
I'd like to go do X with you sometime. Are you free next Wednesday?
Where X is replaced by specific activity of asker's choice. I generally prefer to be asked with an activity and time in mind. I know they aren't particular important, but it shows that someone isn't completely wishy-washy. If I don't like doing the activity or am not free at that time, I'll suggest alternatives. I dislike being asked out where I am then supposed to come up with something to do:
- Girl: Hey wanna get together to do something sometime?
- Phil: Sure, what did you have in mind?
- Girl: I don't know. What do you want to do?
ARG! It's a sign of someone who either has no spine, or can't be bothered to put much thought into things, or both.
This is a cross-post from a crack bored discussion.